It's kind of hot there in summer, but Fruita has a wonderful dinosaur museum, very interactive. An earthquake you can be part of, a dino that spits water, all kinds of things kids love. You can camp about a block away by the Colorado River at the state park, which is very nice.
On your way there or back, stop in Glenwood Springs, where I currently live, and take them on the gondola up to Glenwood Caverns, which is very cool, as well as having some crazy rides, like a swing that goes out over the canyon edge. Also rafting here, which would be perfect that time of year, as well as a beautiful bike path along the river - you can rent bikes. Lots to do here that time of year and it's not hot, very nice in summer. You can also hike to Hanging Lake, though it gets crowded. Lots of other places to hike and see nearby. And a few nice campgrounds, Rifle State Park being one, if you don't mind the 25 mile drive to Glenwood. Or try the Hideout in town if you prefer being here. And kids love the Glenwood Hotsprings and there's also a new one in town down by the river, very classy.