J.P.
We love the Wilderness Resort in WI Dells. We've gone every year since my oldest was about the age of your son, after many people told us it was the best water park for little ones. There are little kiddie areas where they can just sit in shallow water and splash. On our most recent trip, we stayed in a condo rather than the hotel, and it was VERY nice, especially having a full kitchen. They have a great website - check it out!