Anonymous wrote:It's one word: Fallingwater.
Also consider Kentuck Knob.
I want to emphasize what other have said -- those sites are not friendly for small children.
We did Kentuck Knob when my kids were that young by taking turns: my wife took the tour while I played with the kids, then we switched.
Hi honey! Visiting kids to Kentuck Knob could walk uphill to the weird sculpture "garden" (it's a field) and maybe have some fun running around it. They had a British telephone booth up there as well as near the gift shop. (Don't suppose OP's kids are also Dr. Who fans?)
If going full-on intellectual definitely take the kids to Pittsburgh to the Andy Warhol museum!