One of the key factors in your question is that fact that you're talking about July, when it can be scorching. For that reason I would NOT try to do Pompeii. If your g'daughters are into ruins, do Ostia Antica (Scavi) instead. Lots more shade trees and it's only an hour from the center of Rome. (FWIW, technically OA is within the enormous city limits of Rome.)
Orvieto is a good choice because:
1. less than 1.5 hours from Rome.
2. a hilltop town, so more potential for cool breezes.
3. Lots of shady spots
4. A much wider variety of options, as noted above.
For the reason given above, don't do CBR.