help with short Christmas market trip

You will have lots of input on this topic as we just had a healthy chat about it. Opinions are those of the poster. Start with this website that gives you the opening and closing dates of most German markets. www.europeanbestdestinations.com and search for Christmas markets if it doesn’t automatically come up.
Have you researched train times? You arrive in Frankfurt at 1pm and will need time for passport control and customs. By the time you do this, find the train station and buy tickets, your departure could be a good while later.
Check www.Bahn.de for train schedules

Why Dresden? Have you considered Cologne? Another smaller market such as Rudesheim am Rhein or Mainz which are much closer to Frankfurt. I love Rothenburg and its market - very charming town with a couple of nice museums. Have you made hotel reservations?

I am going again this year and spending 2 nights at each of the following:
Rudesheim am Rhein (easy to take a side trip to Wiesbaden or Mainz
Cologne
Valkenburg Netherlands
Munich
Nuremberg
Rothenburg
and back up Frankfurt

Rest easy. You’ll have lots of opinions.

Fun plans! The markets are such a magical experience.

You probably already know from reading the many many posts on this wonderful forum that the trick to the Christmas markets is picking towns with great sights in the them for the day, then enjoying the markets in the evenings.

I would add that I enjoy a late afternoon visit to the markets, when it is less crowded and my photos show more of the decorations in the daylight. I also enjoy taking home the souvenir mugs, so I bring along a box and bubble wrap for safe transport (I sometimes mail my clothes home and take the mugs in my carry on). And, snow on cobblestones is slippery, so I now take a set of removeable ice cleats to put on my shoes once I leave the hotel.

You are not spending too much time in Dresden! It is glorious and magical and the Royal Palace with the Historical and New Green Vaults are incredible (and you should think about booking them now!) and so is the Dresden Molkerei Gebrüder Pfund (look it up!) and so there is plenty to do during the day. You might even want another day there, to help you recover from jet lag.

Nuremberg is also glorious and amazing and it contains two very special museums - the Albrecht Durer House Museum and the German National Museum, both are well worth a visit. Three hours was not enough time for me for the Germanisches National Museum and I am eager to return. Nuremberg also has the WWII sites that you are looking for. Given the length of your trip, you may want to do laundry, and in Nuremberg the Adina Apartmenthotel, which is so conveniently located to both the National museum and the old town, has a washer/dryer combo unit right in the room.

There are some markets open past the 26th. Here's a website with ideas:
https://www.europeanbestdestinations.com/christmas-markets/christmas-in-germany/

Both Munich and Frankfurt are great places to spend time in. Or, consider Berlin - great museums and WWII sights and some markets still open, and fly to FRA for your flight home.

Rothenburg is most definitely worth a night in; the day visitors leave and you can enjoy the less-crowded ambience.

Theater: It's good that you are looking into something else besides Christmas Markets, which despite their small differences are actually similar in most ways. Especially if you aren't familiar with Germany, I would suggest writing your itinerary according to the OTHER things in Germany that might interest you. Perhaps you've done so but not mentioned those things beyond WW II jand theater... Christmas markets these days are everywhere. Do you understand spoken German? Then theatre might be a great option. Otherwise... What other things about Germany are your curious about?

Your plans for Day 1 after exiting your overnight flight are to hop on a train for an additional 4.5 hours or longer. It's not possible to get a good train ticket price (saver fare) for such a long trip on the same day you arrive. And advance purchase of saver fares dictates which train you ride, unfortunately - and thanks to potential flight delays, re-routings, re-scheduling, etc., you don't really know your actual train departure time until you arrive at one of the FRA train stations. Unless you can arrange a flexible but cheap train fare with your airline, two tickets to Dresden at the walk-up flex fare will run around €250. That's just one reason to travel just a sjort distance from FRA on Day 1. The other is the rigor of a red-eye flight, security, baggage, train station, then 4.5 hours before you can drop your things in your room. Perhaps it would be better to visit Frankfurt, or nearby Mainz, for your first night or two. Both have popular Christmas markets. Then proceed on a subsequent day on that long journey to Dresden with a saver fare ticket. Right now, I see a saver fare of €20 each for the 8:14 train on Dec. 18.

Or on the 17th you could travel on local trains to some other nearby destination that gets you closer to Dresden for a night or two. The route between FRA and Dresden is no wasteland, and there are are many nice towns that might qualify for a visit from you. Gelnhausen, for example, is a sweet and attractive town with a market going on your arrival date. You could get there by regional train in just 60 minutes from FRA, and quite cheaply; drop your bags, tend to your personal needs, hit the market, have a meal, see the town, etc., and you'll be that much closer to Dresden as well.

I have no idea if you're interested in cathedrals, Jewish history in Germany, or a spectacular museum like the Technik Museum, but if so, SPEYER is a city you should look into for post-Christmas travel. It's a ways south of Frankfurt and Mainz. The Speyer Cathedral is one of Germany's finest Romanesque Doms. The Christmas market there is open into the New Year. And I believe the Technik Museum is open every day of the year, including Christmas.

https://www.tripadvisor.com/Attractions-g198634-Activities-oa0-Speyer_Rhineland_Palatinate.html

I would look at Salzburg for the 26th - 29th. Salzburg is magical. Some of the markets in Austria stay open after Christmas. If you could change your flight to depart from Munich, it would be easier. If not, Salzburg to Frankfurt us a 5 hour train ride on the 29th.

You are definitely not spending too much time in Dresden. Two full days are barely enough to scratch the surface, especially if you are into museums/art but also love to wander around and just explore the city life. Make sure to walk across Augustus bridge to the Neustadt district - Augustusmarkt along pedestrian Hauptstraße is fun (also the Baroque district west of it with small shops/boutiques) and don't miss the vibrant "outer" Neustadt district beyond Albertplatz square (quirky shops, ethnic restaurants, clubs ...)

Here you'll find all the events that take place during Advent in the Neustadt district:
https://www.neustaedteradvent.de/adventskalender.html
(right now still the schedule of last year, they will update in November)

Unfortunately the very romantic Christmas market in Dresden-Loschwitz ends on 17 Dec, but those in the old town will probably be enough for you to visit in Dresden ... on the other hand, many people enjoy the "Christmas Garden" show in Dresden-Pillnitz (I don't, and tickets are not cheap.)
https://www.christmas-garden.de/dresden/

For Advent concerts check frauenkirche-dresden.de (christmassy organ recital on 18 Dec in the evening.) Nothing in the other main church (Kreuzkirche) but other churches will certainly have something. The Semperoper has a nice symphony concert on both evenings. Annenkirche has a wonderful mass by Johann Sebastian Bach on Monday, performed by an excellent orchestra from Prague.

Another option for Tuesday evening: a cruise on the river incl. 3-course dinner and live piano music. 26 Euro for the boat ride, 26 Euro for the dinner, per person.
https://www.saechsische-dampfschifffahrt.de/en/fahrten/winter-lights/

Do stay in Frankfurt overnight on your arrival day. Taking a long train ride after a long flight, you will be so exhausted the next few days, you won't be able to enjoy anything. Reserve your train for early the next morning and you can take advantage of the cheaper fare and be sure that you will catch your train. Flights lately have been so often delayed. You are also traveling with a 1 year old. Making them sit in a train after a flight is not going to be fun for them or for you.
Arrive, check into your hotel in the central part of Frankfurt and then go out and enjoy the Christmas market. Get to bed early, and take your train the next day.

Things shut down early on the 24th, which is also a Sunday, so you will need to find a hotel serving dinner. The 25th and 26th are official holidays, so stores will still be closed. Museums may or may not be open. If restaurants are open, they will be very busy.

The English Theater in Frankfurt usually has wonderful productions. https://english-theatre.de/

When you come back to Frankfurt, head over to the Palmengarten for the Winter Lights. Your child will love it.

Last fall we arrived in Frankfurt from the US around the same time as you will. Rather than be stressed about making our train connection to Salzburg that afternoon we reserved a hotel near the Frankfurt train station. We also booked a private tour with Frankfurt on Foot; I believe we scheduled the tour for 4:00. It was an excellent use of our first afternoon! Christmas Markets will be open in Frankfurt, so you can make your first visit that evening. (I imagine that you would get some excellent Market guidance during the tour!) The next morning you can easily get to the train station, with your advance-purchase tickets in hand, and head to Dresden.

I spent a weekend in Dresden last December. It has wonderful Christmas Markets and museums. It is close to the Ore Mountains and has a fabulous selection of the best quality German wood carved nutcrackers, pyramids, arches, ornaments, etc. Bring a fold up travel tote for your return journey! We then took a train to Erfurt for an afternoon and overnight. Way off most tourists radar, but a cute town an d a fun market. It has lots of kid’s rides, but a one year old would have to sit on someone’s lap. Then we went to Regensburg which I prefer over Roterburg Ober T. ROT has terrific Christmas stores run by Kathe Wolfhart, but Regensburg seems more real. I love Nuremberg (even if everyone tells you it is crowded) and the Nazi Parade Grounds are interesting.

Ms Jo, what a wonderful suggestion about the Frankfurt English Theater! They just happen to be putting on a production of Something Rotten over the winter months, and also during the stay of the OP, and it's a delightful, hilarious show!

In fact, I just rerouted my own Christmas market trip to these same towns but a week earlier than the original poster, and we're going to see that show! I do love Frankfurt and the Aldstadt, so very excited to see it again. Thank you!