First, while you're at Lake Powell, do not miss Antelope Canyon. There are at least two parts of the Canyon that is on Navajoland and the tribe members leads the tours. We like the first tour on the left side of the highway, DGS and I did that and it is much better than the one on the right side of the highway.
Second, we have taken our DGKs to the South Rim a couple of times and there is a lot for them to do, shopping, the trading post is great, ranger talks and hikes are really cool for the kids. The movies in the visitors center. Don't cut your time too short, go to the VC first and fid out what activities are available.
The reason the average time spent on the South Rim is so short are the tours that bring the ten second tourists. You know the ones, they jump off the bus, run to the edge, take three photos and run back to the air conditioned bus. Ten Seconds.
Don't forget that there really isn't much to do at Dead Horse Point. It is an overlook but has a nice picnic area. I don't remember if it has water.
Canyonlands has three distinct areas, Island in the Sky and the canyon below DHP. The Needles area and the Maze area. You will only be going to Island and Needles. Arches is a BIG draw and there can be traffic jams. I don't know if the road improvements have been finished and I no longer have any idea about accessibility.
You didn't mentioned Monument Valley, on the way to CYNP from Lake Powell. Lots of beautiful landscapes and movie history.
Have fun and let the kids learn as much as possible from the staffs at the parks.