We took the trip at the end of June last summer with our then 10 year old daughter. This was a second
ABD trip for us after having done Knights and Lights the summer before. Overall, the trip was fantastic.
First of all, make sure the Disney people know you have a birthday. They will make it special. Getting in a day early is good but try not to sleep during the day to get adjusted to the time zone. This trip has a lot of hotel changes so it helps to pack in a way that minimizes emptying and repacking each time.
I won't spoil the disney agenda but there will be lots of things to do on your own. Your extra day can be spent leisurely walking the Old Town in Heidelburg. Check out the Hauptstrasse for shops and restaurants.
Plan on eating a lot of schnitzel but we had no problems keeping our finicky eater happy. Be sure to try some strudel at some point.
Kid stuff abounds and lots of special surprises that I can't reveal. You will stay at a real castle in Trendleburg. The activities include archery and bike riding. It was a big family day. Make sure everyone is up on their bike riding skills. The Steiff factory was also a big hit. Shopping in Rothenburg was very cool. This is the best place to buy your German souvenirs. One shop keeper (written up in Rick Steve's Guides) was extremely kind to our daughter. The shop is Anneliese Friese.
Bring activities for down time on the bus trips (books, gameboy, etc.) Munich was fun all around. Bring a bathing suit as all the kids hit the pool during the Munich stop.
All in all, the itinerary was heavy with castles, princesses and fairy tales. The guides do a wonderful job of keeping every child engaged and happy giving the adults a chance to enjoy themselves too. You won't be disappointed.
Have fun.