The Everglades, take 'em on an airboat ride. And there's always the Keys...don't forget the Skin-so-soft. Bugs and big lizards, what more could any six year old ask for? Or photographer for that matter.
Another vote for Yellowstone, also the Smokies are close and the wildflowers should be putting on a good show. If California is doable, I'd suggest Death Valley, Mono Lake, and Bodie. Hwy 395 is a fabulous drive in spring. If the ocean is more your style, start in Laguna Beach and head north on Hwy 1. Stop where you want, as it goes to the Canadian border. I'd go at least as far as the Olympic Peninsula in Wa.