Grand Canyon and Moab regularly see 100 degree days in the summer. We are from Texas and would only consider these places outside of summer.
We have not spent time at the Grand Canyon, but have visited the Utah national parks twice---once in mid March and once in late April. Both times, we'd start out in winter coats for early (7 or 8am) hiking but would quickly warm up from physical activity and the day's temperature. By lunchtime everyone would be in long-sleeve tshirts only. (I would also recommend coats if you take a springtime boat ride near Canyonlands NP bc of the boat wind.)
It would be difficult to imagine enjoying much of the outdoor activities in Utah in summer 100 degree temps with the exception of whitewater rafting. It really is an easy area to do OYO, but full disclosure we are not group tour national parks people. They are easy trips to research, safe places to travel, and we enjoy the flexibility and tranquility of hiking, wildlife spotting, etc. on our own vs. with a big group. So...for our family we couldn't imagine enduring brutally hot temperatures when you can have a (better to us) trip OYO at spring break time.
However, y'all might have completely different preferences on national park travel. If doing the GC/Utah trip with ABD is a huge preference, I'm sure you'll still have a wonderful trip and the scenery (being mostly rock formations) remains great year-round.