If you want to be a hero to your kids, take them to the baths of Budapest.
They will love the wave pool at the Gellert Hotel.
The Szechenyi Baths in the City Park on the Pest side are also very kid friendly. The have a circular jet spray (don't know how to describe it, but kids love it).
There's a zoo, a railroad museum, and a fantasy castle adjacent to the baths.
Also, consider the caves in the Buda Hills. The longest is called Palvolgyi.