The thing about summers in Tucson is it can be hard to leave the house. In Tucson—especially for those of us looking after kiddos—it's important to be near a pool. Leave some ice water at the edge of the pool, do a few cannonballs then retreat for a cool shower and a snack. Nothing says "Tucson summers" quite like keeping close to the AC.
Runners up:
2. Children's Museum
3. Breakers Water Park