Germany-Where should I be going?

In Germany, Dec 25 and 26 are holidays - so things will shut down the afternoon of Dec 24 and not open till Dec 27.

Study seat61.com to learn about trains and how to book - lots of great info there. Always book on the official German site, BAHN.

I don't know much about the area around Frankfurt as we prefer to fly into Munich and see the area around there (lots of castles, Regensburg, Nuremberg, etc).

There is a lady on these forums who lives in Frankfurt and she is very helpful, so I am sure she will chime in soon.

We like Heidelberg a lot, but I'm not sure how far it is from Frankfurt. And very few Xmas Markets stay open after Xmas - hoping the lady on here from Frankfurt can help you out.

Website of Deutsche Bahn: https://int.bahn.de/en
Trains and local transport are traveling also at Christmas.

Streets in towns are usually well lit.

"We are looking for small picturesque towns that have a german vibe rather than big city, oh and castles would be nice too."

To avoid getting stuck in your room after sundown you should probably look at "mama-bear"-sized towns rather than the truly small ones.

KOBLENZ could be a good base town for a few days. It's a smallish city with plenty of restaurants and services as well as a rail hub town; it's located about 90 minutes NW of FRA airport by direct train (9:24 - 10:54, for example.)

Railway map showing FRA airport, Koblenz area:

https://www.seat61.com/images/rhine-train-map.jpg

Koblenz is home to separate Christmas venues; some remain open into the New Year:
https://www.koblenzer-weihnachtsmarkt.de/en/oeffnungszeiten

Koblenz is only about 10 train minutes from the small old-world town of Braubach, the home of Marksburg, a completely intact medieval castle with tours available year-round. The town and castle make for an easy visit from Koblenz:

https://www.marksburg.de/en/circuit/#/
https://fotos.schloemp.eu/wp-content/gallery/braubach/002Braubach-Bauernschaenke-Eck-Fritz.JPG

Koblenz has an aerial cable-car ride that takes you across the Rhine River to Ehrenbreitstein fortress and the Christmas Garden:
https://www.seilbahn-koblenz.de/homepage.html

Other destinations, small and medium-sized, that are easy to reach by train outing from Koblenz:

Cochem
Trier
Cologne
Bonn
Limburg
Bad Ems
Rüdesheim
Mainz

You will not find restaurants available on Christmas Eve, Christmas Day, or the 26th. That said, the major train stations in the area (Cologne, Frankfurt) and perhaps certain big-name hotels will have meals/food available.

Both the Christmas Market and the Winter Gardens will be open in Koblenz after Christmas, so you may want to look at this as a base. But I'll recommend Heidelberg instead, as it's an easy train ride from Frankfurt, some of the Christmas markets will still be open, you have a really good castle, and it's touristy enough that you can probably find a place to eat Christmas day. Plus from there you have a number of easy to reach destinations that fit the "small picturesque towns" criteria. Bad Kreuznach, Neustadt (winestrasse), Bad Durkheim, Speyer, Neckargemund (with 3 castles), etc. You can even take a train to Stuttgart and tour the Porsche or Mercedes Benz Museums (although this would be an ICE train and you'd have to buy separate tickets).

Heidelberg is well lit at night, and the long "car free" zone is jammed with shopping and places to eat. Plus it's a University town, with a large international student population, so your teen should be able to meet and talk with like aged individuals.

You can tour this whole area on the Germany Pass, it also give you the in city trains and trams, and you could also use it to go up along the Rhine (but that area is really dead between Christmas and New Years).

There will be places to eat open 24, 25, 26 DEC, but not many. You should put some effort into researching this. If you look around you'll find places. For example a quick google search says the Heidelberg Schloss will be open both the 25 & 26th. You should also check out the Michelin guide. (I had a fantastic Christmas meal last year at the Meissenheim Hof, a Michelin restaurant.) You want to make reservations at least 30 days out.

Christmas Eve is the tough one as just about everything is closed by 1400 if they were even open at all. The 25th and 26th are popular days for restaurants and many museums are open too. You may need reservations for the restaurants though as they will have a holiday menu and be full!
Yes, the towns are well lit at night and it will be dark by 16:00, not 18:00. Why would you stay in your hotel room after dark? Christmas decorations are all up and people are out and about shopping, etc. except on the holidays. Germany is safe at night, this is not something you need to worry about.

If you land on the 22nd, you could visit the Christmas market in Frankfurt as that is the last day and traveling after a long flight is not a lot of fun.

These markets in the Rhein-Main region will be open when you are here;
Limburg Christmas Market – 15.11-5.01
Worms Christmas Market – 20.11-23-12 (closed 24.11)
Rüdesheim Christmas Market – 21.11-23.12
Aachen Christmas Market – 22.11-28.12
Koblenz Christmast Market – 22.11-05.01 (closed 24.11)
Frankfurt Christmas Market – 25.11-22.12
Wiesbaden Christmas Market – 26.11-23.12
Mainz Christmas Market – 28.11-23.12

If you are in Koblenz, check the opening days for the Christmas garden. Riding the gondola up there and back is stunning. Ride at sunset.

Russ is right in suggesting staying in a decently sized city instead of a smaller town at that time of year.

we stayed 3 nights in Bacharach and even the tourist office was closed.

Such great info from all of you, thank you so much. I am going to look into Koblenz. The insight is what I am looking for and I am glad you all chimed in. Thank you so much!