It's hard because every city claims it has the best market. It's almost a status thing in Germany!
Basel has a lovely one, and they have a separate market for kids with crafts and trades to learn.
Also the one in Colmar, France has been recommended to me to try. I've been to the Montreux one which was okay and there are other things to do on the lake but eyewatering in terms of cost.
Nurnberg also has a kids section but it is quite infant based and the market gets to heaving levels with tourists and very drunk visitors. It is packed in and at one point, trapped in a sea of pushy shoppers and cheapo plastic baubles I wanted to cry....
Gothenburg - Liseberg (pronounced Lisaberry) has the most beautiful market I have been to, 1 million white lights and it's in a theme park. You can fly on the cheap from Ryanair Stansted.
Stalls are very repetitive. I know this is way off the plan, but why not take her to Christmas in Europa Park, it is amazing, there are rides, a Christmas market, Circus, Ice show etc and the hotels are amazing and there is still the obligatory Gluhwein. you could drive or fly into Basel.
Try posting on the Kaffee thread in living overseas for some local info.
DD has nearly two weeks holiday before christmas this year and as she will be 10 in Jan I suspect the christmas magic will not be with us for much longer. So I am considering a short trip to a Christmas market. Which would you recommend for a 10 year old? We are reasonably adventurous we happily travel in any medium (we returned overland from Oman in the ash cloud). It will just be her and me. We have considered Eurostar to Paris, night with friends, sleeper to Munich night in Munich and then on to Salzburg and then back via train. I know christmas is along way away, but for a variety of reasons I need to organise early.