You should definitely check out the aquarium, as it's nice and a good way to get out of the heat. The zoo's pretty nice as well, but hot as hell in the summer.
Take 'em to Cafe du Monde at least once for beignets for breakfast. Exploring Jackson Square is always nice, and you can take 'em to the French market for cheap souvenirs.
I've never done it, but I've heard the Steamboat is a pretty cool thing to do. You could also take them over to the Mardi Gras world where they have all the old Mardi Gras parade floats. It might be of interest to them.
I've not done the family-friendly New Orleans trip yet, so I'm sure there are those around here who will have far more advice than I. Now, if you want suggestions on meals and late-night debauchery, I can help you there