I agree with BB TX. Three or four days is enough. When I took my brother's family with my 12 yr old nephew to the Grand Canyon just for a day trip, after the third overlook, my nephew said "Do we *have* to stop at *another* overlook to see the same big hole?" He's a city boy. YMMV. There is a junior ranger program at GC. And there is an IMAX theater in the nearby town of Tusayan...where, by the way, there is another campground with hookups (Camper Village), in case you can't get reservations inside the Park.
Yes, you could combine your week with Bryce, also at high elevation so not so hot in the summer. But then, depending where in "Ca" you are from (if that's California)...you may as well drive through Zion on the way back home.
Alternatively, you could add Sedona, but warmer in the summer than GC. Or Flagstaff (higher elevation, similar temp to GC) where you could visit Walnut Canyon Nat'l Monument, Sunset Crater/Wupatki NM and loop road, Lowell Observatory, Museum of No. AZ. You could do a day trip down to Sedona from Flagstaff through Oak Creek Canyon.