My top five National Parks for children of every age, without regard to the cost of getting there, have easily viewable wildlife and "active" features--geysers, glaciers, volcanoes, waterfalls, etc.--or ancient American cultures:
Yellowstone, WY (herd animals, bears, wolves, geothermal features, waterfall)
Kenai Fjords, AK (sea otters, whales, sea lions, sea birds, tidewater glaciers)
Hawaii Volcanoes, HI (humpback whale watching and active lava flows)
Katmai, AK (brown bears, salmon, volcanoes)
Mesa Verde, CO (climbing through 1,000 year-old cliff dwellings)
My grandsons still talk about a trip to the Big Island of Hawaii, their whale-watching boat trip, and seeing the lava flows pouring out of Kilauea Volcano at HV NP. They were 5 and 7 at the time.