I'm not sure on the best Denver/YNP route since we normally go in via Salt Lake City to West Yellowstone. I would think some time on I-25 (once you get out of Denver traffic) would help getting used to the RV. Settle down in the right (or middle if available) lane at around 60mph and let it run. Don't let anybody push you around or crowd you. They have your permission to go around you and go as fast as they wish.
Do you have your reservations at Yellowstone? It is still very crowded at the end of August and choices may be very limited at this time although you need a much smaller space than my 54 foot combination.
Don't miss Dinosaur, The kids will love it. Plan to spend at least half a day at the fossil face.
Last Summer, with the grandson, we left Yellowstone via the South Entrance; down through Tetons to Jackson Hole; then to Green River, WY (overnight stop?); next day down through the Flaming Gorge Rec Area (take it slow and easy. Lots of scenic pull offs.) to Vernal. You would be going the other way. Vernal; Flaming Gorge; Green River; Jackson; Yellowstone.
The KOA at Vernal is nice, if a little pricy. Very friendly people with lots of local knowledge. You older one would enjoy the Petroglyph Ranch, but probably not so much for a five year old.
Have fun!