Interseting travel tipps, however I would not forget that two boys are going to be bored with mainly the 'adult' travel tipps.....
My thoughts:

LEGOLAND - which you mentioned allready
In SINSHEIM there is a huge technical museum with a real CONCORDE, BOEING 747, an many other airplanes, etc.
STUTTGART - Visit the Mercedes-Benz Museum
If you go up north on the Rhine and make it all the way to COLOGNE, there is the PHANTASIALAND (kind of Disneyland, just MUCH smaller)
And the Castle NEUSCHWANENSTEIN (Cinderellas Castle) near Fussen at the Lake Constance (BODENSEE) has been mentioned before...

All the other stuff is great to do, however the boys would become bored soon and then.....it's no fun for anyone

Enjoy GERMANY