Yellowstone & Grand Teton next summer, from Colorado Springs

Unfortunately no great advice for you on any stopover points. FYI you can rent kayaks at Colter Bay.
Looking down the road, I would also consider a trip to the Black Hills. Besides being very scenic, it's very kid friendly with lots of "touristy" things to do. Our kids liked it better than Yellowstone at that age. By the way Yellowstone is huge, expect to spend a lot of time driving after you get there.