Arizona/Utah or Yellowstone with young kids

[CJinMD]

We are looking into doing our first ABD trip. My boys (at the time of travel) would be 8 and 6. We would really like to do one of the Yellowstone trips but the "suggested age" for most of those trips are 8, so my younger son wouldn't be able to do some of the planned activities (like horseback riding).

The Arizona trip suggests age 6 - has anyone done this trip with younger kids? Is there enough "activity" to keep them interested? Im worried it will be too much sightseeing and long bus rides for them.

Any insight would be greatly appreciated!