We did a trip a few years ago when our kids were 8, 10 and 14 in July. It was HOT - it doesn't matter if it's a dry heat - 115 degrees (Hoover Dam) is hot. Just prepare for it - tons of water, those cool neck wraps, hats, etc. We had an amazing trip - the boys still talk about it.
We flew into Phoenix and then spent a week in SW Colorado (Durango). Did Four Corners, Mesa verde NP (my favorite), whitewater rafting, Durango-Silverton train, hot springs in Pagosa, Sand Dunes NP, horse back riding and a few other activities. We stayed in a timeshare for the week.
The 2nd week we traveled and spent each night in a different place - it was exhausting, but worth it. Stops included Colorado NM (another fav), Dinosaur NM (Utah), Arches NP (Utah), Las Vegas, Hoover Dam, Bryce NP (another fav) and Zion. For Bryce and Zion we stayed in the parks which I highly recommend.
The 3rd week we had another timeshare in Sedona. Slide Rock SP was awesome, did day trip to Grand Canyon (were actually not thrilled after having seen so many other wonderful sights!), Meteor Crater (a tourist trap but DS10 requested it and we're glad we did it), Sunset Crater Volcano, Canyon de Chelley, Petrified Forest and Painted Desert NPs and did a Hummer tour.
I think I missed a few things, but that's most of it. We were busy and it was hot but we'd do it again in a heartbeat! I used Trip Advisor like PP mentioned and got a ton of great info.
Do it - you won't regret it!!!