I'm not sure if you know about the National Park Passport stamp program. Each unit if the National Park system (401 total units) has a unique stamp that has a changeable date. There can be multiple stamps at the different visitor centers. Especially Yellowstone, there are many different stamps there.
They sell a Passport Book (around $8 or so) and it has space for stamps from each region. This would be a fun thing for your kids to collect at each park. Once you have the passport book, the stamps are free at all the parks. Then they have a record of the date they visited each park.
You'll actually be passing Jewel Cave National Park on the way to Yellowstone. And also Devil's Tower National Monument. And if you want to stop by them, there are several others on your way back to Edmonton. There's Little Bighorn National Battlefield, Big Hole National Battlefield, Bighorn Canyon National Recreation Area, Grant Kohrs Ranch National Historic Site, and even Glacier National Park. I believe that all of these sites have a "Junior Ranger Program" where the kids get to do a bunch of fun activities and earn a Junior Ranger Badge, a certificate, and sometimes a patch or other item from the park.
I am on a quest to visit all 401 units and collect a stamp from each one. I am at 353 now. I sure wish I would have started collecting those stamps when I was a kid.