That's an ambitious itinerary especially with two small kids. Yellowstone will be very crowded at that time, and traffic will be heavy, both because of people and because of wildlife along the roads, which will cause people to stop and watch. What's more is that very little of what there is to see can be seen from inside a vehicle. You are going to want to get out and look around.
It's not hard finding diesel fuel anywhere although there are places a long way to the next stop. I too start thinking about tanking up when the gauge hits half. If you don't already have the app GasBuddy, get it. It will help you find fuel and avoid being gouged.
Have a great time.