This is one of the areas in Germany I know as a tourist only. And as a tourist I would think of doing the following:
- Take two days in Heidelberg
- Drive along the Rhine downstream and have a night in one of the castles
- Drive upstream in the direction of Switzerland
- Cities to visit would be for me Strasbourg, Colmar, Tübingen, Stuttgart, Rothenburg, Baden Baden, Freiburg (and your Legoland)
- Tour for a day in the Black Forest. You could take the cable car up Schauinsland Mountain.
- Drive along Lake Constance (Bodensee). Have a stop at some of the villages there. Meersburg is a fairytale village. A flight with a Zeppelin (http://www.zeppelinflug.de/pages/E/haupt.htm) could be a highlight for your kids and is really something to write home about.
- Continue to Zurich and to central Switzerland (if your kids collect "countries" you could make a visit to Liechtenstein, one of the smallest independant countries in Europe and certainly worldwide). Luzern is nice city in central Switzerland with plenty of mountain railways. This is Switzerland as you know it from any movie about Heidi and friends.
- Drive back via Salzburg and Munich.
I hope locals can give you some more educated ideas, there are a few on this boards.
Cheers
Thomas