J.A.
I grew up vacationing in the national parks and I worked in Acadia for 3 summers in college. Either park is a winner with lots to do with a 7 yr old. Yellowstone is bigger and more isolated, Acadia is a short drive to a walmart. I know Acadia has a bus system to see the park and island, I don't know what Yellowstone has now (It has been close to 20 yrs since I was there last). I know park lodging and some campgrounds fill up very quickly so the earlier your reservations the better. Yellowstone has lots of large wildlife to see, I remember walking past bison on the sidewalk! You'll have fun wherever you go, Have fun planning!