The water is colder than the San Juan, it's only 80 or so miles from the mountains and the San Juan has a long trip south to warm up before it turns north into Utah.
With kids that young I would stick to the Slickrock-Bedrock section. Easily stretched into 4 days if you want.
The Gateway-Dewey section is also fun, but the shuttle is long. The scout at Statelins is river right, but the dirt road to walk little kids on is river left, that may cause problems.
I frankly had more trouble in the Boulder Garden downstream from Stateline because I wasn't really paying attention and i nearly got swept into a wrap rock the size of a house.
Both are beautiful trips, te gateway trip has a graded dirt road parralelling the river for the fist 13 miles from Gateway, well past the Stateline Rapid.