Ah yes....camping with little ones! I remember those days, my kiddos were the same age apart (3 years) and about that age when we were taking them on camping trips in our tent camper.
So. Activities for the kiddos and FHU campground/RV park. For kid friendly camping, it's hard to beat Colorado Springs as a base for activities. There's an abundance of things to see/do that your children may enjoy. Things like:
Cheyenne Mountain Zoo
Pikes Peak Cog Railway (in Manitou Springs)
May Natural History Museum
Cliff Dwellings and Cave of the Winds (close together)
Cripple Creek and Victor Railroad (up in Cripple Creek)
The Canon City/Royal Gorge area (about 45-50 minutes SW of Colorado Springs)
This is just to name a few off the top of my head.
H E R E is a link a more complete list of things to see/do in that area. You may want to consider
Mountaindale Cabins and RV Resort outside of Colorado Springs as a base camp location. It's a bit out of town, but easy to get to various sights/locations mentioned.
The previous suggestions from others are also very good recommendations. Just depends on where you want to be and how much you want to see, in whatever time you have allotted.
Great Sand Dunes NP is a wonderful place to visit.
H E R E is a campground just outside the park that could work for you. There's also a KOA in Alamosa, a bit of a drive, but a good base camp location.
Either place to stay, you could consider going down to Antonito and doing the
Cumbres and Toltec Railroad. Did that with my kids (when they are just a bit older than yours) and they loved it. There's also an alligator farm near the Great Sand Dunes, that could be something different for the kiddos AND the parents.
H E R E is the link to them.
Whatever you decide, make sure you get your reservations going ASAP. You will need them.