1st time in Munich-Traveling with 35 yr old, 6 yr old, 60 yr old and two 70+ yr olds

Rothenburg really is too far for a day trip. You could spend all your time on the train. Regensburg, Augsburg or Landshut would each make a nice day trip by train. I would still take a taxi to city center from the train station for any of these due to the walking issues.

I thought my must-sees were Neuschwanstein and Rothenburg de tauber but after reading reviews, I am not so sure.

These both include a significant amount of walking so I agree with your thoughts on rethinking it. Plus Füssen or RoT will burn an entire day and for RoT you'd spend more time getting there and back than you would probably spend in the town. There is plenty to do closer to Munich. In addition to Landshut or Regensburg, you could also travel to Salzburg easily as a day trip or spend time in the lake district south of Munich in the foothills of the Alps. There are a few forum members who really like Ammersee and I prefer Tegernsee area. Both have easy access from Munich (S-bahn or BOB train), good food and beer, boat rides across the lake, nice walking/cycling paths, and great scenery.

Are there bike rentals within English Garden?

There are not but Mike's Bikes rental is pretty close to the southern end of the park so you could just rent there and ride to the park. Call or email them and see what they suggest for a 6 year old.

We are not renting a car but would like to do some side trips (but renting a car could be an option.) I would be the one driving so I'd like to know how difficult it is navigating.

No, driving is not really difficult but in with only three days and with your hotel already booked in Munich it would be a hassle. There shouldn't be anywhere that you might want to go that is not easily accessed by a combination of train/bus/taxi.

DJ

Since you are unfamiliar with Germany, I think spending a half day of your limited time exploring Nazi stuff (which is maybe the only stuff Americans might know about Germany in the first place, thanks to our film industry and our education system) could be a misstep that only stereotypes.

Munich's a big place. You might have time for ONE side trip. There are a lot of good destinations but they tend to involve some long walks. I'd suggest a visit to the mountains. Mittenwald is an adorable village that doesn't require any hiking to enjoy the mountain scenery and that you can reach in less than two hours by DIRECT train (no changes in other words) - leave Munich's main station at 8:32 or 9:32. Small town, easy walking and low-level sightseeing, plenty of cafes. You can reach the Karwendelbahn lift easily on foot. The Leutasch gorge is a bit further out. Here's a good online guide.

When you return to Munich stop over in Garmisch-Partenkirchen if you have the time. The Fraundorfer Inn (Gasthof Fraundorfer) is a fun place for traditional Bavarian meals; in the evenings there is music and traditional dancing.

There's a day pass called the Regio-ticket Werdenfels which you can buy at Munich station ticket machine for this trip. It covers 5 people for €44 round trip. You can depart Munich at any hour of the day (no hours restrictions as with some train day passes.) I don't know of a current page in English that explains it. Here's the German page.
https://www.bahn.de/p/view/angebot/regio/regionale-tickets/bayern/regio-ticket-werdenfels.shtml

Thank you all so much for your help!

We loved the Third Reich walking tour we took through Munich Walks. It was about 3 hours, but did include a lot of walking.

http://www.munichwalktours.de/en/hitler%e2%80%99s-munich-third-reich-tour/

I recently spent a long weekend in Munich and we took a day trip to Salzburg, a very easy drive and a fantastic city. I'm sure everyone will enjoy Hohensalzburg Castle and there's a funicular to the top to save on climbing all the steps. I'm sure the 6yr old will be fascinated by the museum full of medieval torture instruments! You don't say when you're going but in the summer there's nothing better than sitting at the top of the castle, a beer in hand whilst looking out over the alps.