Denver Yellowstone Loop

[babyruth]

I've been reading through many of the threads here over the last few days and could use some advice on planning a trip that will most likely start and end in Denver. SLC could be an option but is more costly to fly into w/ less days. I'll be traveling w/ 3 boys (2 teens & an 11 year old) who although active are city kids who didn't love our ambitious hikes in Yosemite a few years back - so all hiking will probably be on easier/shorter trails. Some sites I would like to visit are Yellowstone, Grand Tetons, possibly a trip over to S Dakota & the Badlands and maybe some sights in Utah. My 16 year old would like to see Montana as well. Any suggestions for a 12 to 13 day loop? It doesn't have to cover everything mentioned rather these are just the sites on my wish list. Thanks!