Just did the exact same np tour. Are you hikers? If yes, then 2-3 days per park. More days obviously lets you hike more. GC down to river and back up takes two days itself. If like us you are not hikers, then you can do 2 days GC, 2 Zion, 1.5 Bryce,, 0.5 Arches, plus other stuff at Moab. There is not much at Capitol Reef. We skipped Canyonlands so no idea on that one. By that point we had seen enough canyons, lol.
I agree with a post above that if you go quality over quantity, then skip Moab and maybe spend time on a river or lake on the way home. I doubt if your kids are gonna care for Silverton or Durango unless you do some rafting, kayaking, jeeping.