Are you living in Texas?
We've always found Colorado Springs / Manitou Springs to be fun with young kids. Cooler temps, lots of outdoor activities and our kids favorite at that age was The North Pole at the base of Pike's Peak. It's a Christmas Themed park with just tons of rides for kids of yours' age.
https://northpolecolorado.com/ Garden of the Gods is a lot of fun too.
Dinosaur Museum in nearby Woodland Park if your kids are into Dinosaurs
https://www.rmdrc.com/Olympic Museum is great.
At that age I feel like you are just trying to enjoy spending time with them someplace new so make it easy on yourself, right?
I wouldn't do Yellowstone or any of the "big parks" at that age as none of your kids will really remember or appreciate it just yet. (When they get older make sure to do the Jr. Ranger programs in all the National Parks.)