Flaming Gorge is a great place to boat/camp/fish. The kids would really enjoy cruising around the lake on a boat. If you are camping on a beach, having a boat available is really nice.

The fishing is really good on the lake or in the Green River below the dam, but it might be beyond the patience/skill level of your young kids. If they want to catch a fish, perhaps you could go over to the ponds next to the Red Canyon Lodge.

There are also plenty of places to go hiking in that area, and your kids might also enjoy taking a tour of the dam.