traveling West on I 80 to Yellowstone with a 4 year old.

some of these folks are getting I 90 confused with I 80.
I don't have much to suggest for a 4 year old. Most 4 yr olds are happy just getting out of the confinement of seat belts, etc and running around. Take a soccer ball to kick around in city parks. NE towns all have a city park of one kind or another, some with swimming pools and kids' playground stuff. These will be better than the interstate rest areas and are usually easy to find.