Oregon Trails in Baker, Or. Check to make sure your dates are not during the Fair.
Next stop would be at Craters of the Moon. A little educational. It could be very hot though.
These two stops would break the days down.
Think about reservations in West Yellowstone. Could be too late to get sites reserved in the park.
With kids, I would suggest staying in West Yellowstone at the KOA. It has a great indoor pool and other things for the kids. They have some good breakfasts and dinners during the season to save doing in the rig.