Most of your children are too young to enjoy this trip or remember it. I'm not saying to not do it...just an opinion.
The biggest problem I see with your trip is it's going to be hot hot hot. I would spend more time in CO. I am not a fan of the south rim with its eight lanes of traffic going into the park, especially in August. The north rim will be cooler as it's 1000 feet higher and there's only one lane of traffic into the NP. I would change your route to Colorado Springs or Salida, Mesa Verde, Durango/railroad, Page/Antelope Canyon, and then the north rim...decreasing the time in Page.
From Bryce I take 22 north and do the southern part of 12 as a day trip Including Kodachrome from Bryce.
I would take the kids to the springs in Glenwood. X3 or is it X4 on Grand Lake instead of Estes Park.