As mentioned above - the Autobahn is perpetually under construction - so watch out for delays (and the German Drivers). The Deutsche bahn (www.bahn.de) offers deals for parents with kids (kids are usually free with a paying adult) and may be a nice break from all that driving.

Lastly, not sure if you've thought of this - but the new "fast train" that travels about 350 kph (220 mph) can get to Paris in 2 hours 30 mins from Kaiserslautern (45 mins from Heidelberg). A little bit faster than the rides at Legoland and I'm sure you can find something to do in Paris.

http://www.ramstein.af.mil/news/story.asp?id=123057519