So with your kid's ages in mind, here's some ideas:
Head to the four corners area (CO, UT,NM, AZ). For southern Utah, maybe get a campsite at Watchman CG in the park. Electric hookups only, but there's a great stream right by the campground that is wide and shallow. Kid's can spend hours building little dams and wading beneath the cottonwood trees. Good way to keep cool during the heat of the day and it's a gorgeous place for the parents to take a couple of chairs down to the creek and watch. (This assumes you're not travelling in March and April when the flow can be high). Zion uses a shuttle bus system to access the canyon that leaves from right near the campground. There are lots of shorter hikes the kids will love (plus the excitement of riding the bus). If you watch kids, they tend to focus on smaller, up close things than taking in big scenes. For this reason, I'm not sure they'd get nearly as much out of a trip to the Grand Canyon as they would Zion. Likewise, a 4 year old can't take very long hikes either. (And they can be worrisome near the rim of a cliff.)
Down in the same area, you can also ride the Durango-Silverton train I mentioned. Take another day at Mesa Verde and do the short, paved path to Spruce Tree ruin. Explain to your kids that kids their ages lived there about 800 years ago. This is a place where they can get an education without even knowing they are!
Next, drive up to Ouray. It's a gorgeous drive through scenery that would probably be a national park except for all the mining. Take the kids to the Ouray hot springs pool. It's our favorite hot springs pool in the state (shh, we live in a town with a famous one - what a turncoat). Great scenery right from the pool and there's a great free slide for the older kids. When we were there a few years ago, we sat soaking in the pool while a bear was on the hill across the road foraging for food!
From Ouray, head east on US 50 to Salida. You can stop at Black Canyon of the Gunnison NM if your schedule permits. A really dark CG where your kids will see more stars than they ever have in their lives. Deer often wander through in the daytime. If you get the time, drive down the East Portal road that takes off from right near the entrance station. No place to go with your trailer, but take the truck down. You'll end up right at the bottom of the Black Canyon looking up.
Spend a couple of nights at Salida. Head up to St. Elmo, a real ghost town. Across the street from the little store you'll find dozens of squirrels and chipmunks just looking for a handout. Hummingbirds all over the place. Great old wooden sidewalks. The older kids might enjoy an easy float trip. Salida has a great downtown with a park right on the river. Interesting art shops. Check out
this one. Don't you think your kids would get a kick out of this shop?
From Salida, head south to Great Sand Dunes NP. The CG there is quite nice. If you're there in June or early July, Medano Creek may still be flowing. Sand dunes, a stream to play in, cool showers to rinse off in - it's a kids giant sandbox/playground!
Seeing the sights I've listed above might take 10 to 12 days out of a three week trip. So that leaves five days each direction to get out here and back home. That's 400 miles a day. Make it a couple of longer days interspersed with a couple of shorter days. I don't know where you are in FL, but maybe a stop near one of the MS beaches? It is tough, as any time you are talking about going across much of Texas, that's a day or two! But getting to the four corners region is probably a day shorter in each direction than Yellowstone via the Black Hills.
When you get back from this trip, you'll have seen a few NP's. Will you be able to brag about all the parks you went to? Maybe not. But you'll be able to brag that your kids had the most fun on a vacation ever.
