If your kids are dog fans, I highly (and I mean highly) recommend a little side trip to the Denali Park Kennels (free!). Most folks don't realize that the Park is still patrolled by dog sled in the winter (a combination of nostalgia and the fact that dog sleds are more reliable than snowmachines in those conditions) and maintains a large kennel year-round. The dogs are chosen based on friendliness and athletic ability, and love to have visitors. You can walk right up to the dogs and love on them, and a couple times a day, the rangers will put on a demo and kids can ride the sled (on a gravel track) behind some dogs. It's really a neat experience for the dog lover-- expect to leave covered in dog kisses/spit.
Also, if you go at the end of August, you can take the kids to the Alaska State Fair in Palmer-- very kid friendly (lots of critters, some rides, arts and crafts) and a really mind blowing display of giant vegetables (40 kg cabbage!). The summer is short, but ~20 hours of daylight/day is evidently very conducive to growing world record veggies. There are typically some decent musical acts as well if your kids are in to that sort of thing.