I think your plan sounds fine since you have been north and in Colorado in previous years.
You can easily drive from the Moab-Arches area to Black Canyon of the Gunnison NP in a day and still have time to see the canyon before dark. Next morning you are back on your way east and will have avoided going through Denver. Then you could end the trip by staying on US 50 and taking that Arkansas River raft trip or get on CO 114 to the Great Sand Dunes for some sand-sliding down a 700-ft. high dune or two. Either way, the focus on that last day in the West will be on something memorable for the kids.
Of course, if you did both of those things when you came to Colorado previously, then it's a moot point!