If you can get in, Devil's Lake State park in Baraboo is only about 20 minutes south of there. And Devil's Lake by itself is a great attraction. The other parks nearby are Rocky Arbor (just north of the Dells) and of course Mirror Lake. Check RV Park Reviews - Trusted Reviews of Campgrounds & RV Parks
for private CGs in the area. We've only stayed at one private in the Dells and they had a "no kids" policy.
X2 on the suggestion for Circus World. Also take a WI Dells Duck boat tour. Well worth it because the river is gorgeous. Tommy Bartlet's Water Show is really good too. Ho-Chunk casino about midway between Baraboo and the Dells has a really good Las Vegas style buffet. About 30 minutes or so southwest of Baraboo is The House on the Rock
. I can't even begin to describe that place other than to say it's amazing. While pricy, Noah's Ark water park is a great place to spend a day.
Avoid most everything else in the Dells except maybe the go-carts. Very commercialized, over priced attractions that leave you feeling empty when you are done.