For toddler aged children, I'd recommend the Hilton Hotel's waterpark in Milwaukee. It is ideal for younger children (infant through age 8 or so), as it is far calmer than the pools in Wisconsin Dells (we used to take our 6 year old daughter to the Wilderness in the Dells, but it is a long drive and enormous and a bit overwhelming for small children, as were the other waterparks we checked out in the Dells--the Kalahari and Great Wolf). We took her to the waterpark at Lake Geneva once, and swore we'd never go back--too many people in too small an area, so the indoor park area is always loud with older kids racing by younger ones who are easily knocked over (I believe they let people in by the day, as do many of the waterparks in Wisconsin Dells, so things get crowded quickly).
The Hilton's waterpark in Milwaukee is an easy drive, and when we were there for a weekend about three weeks ago, we praactically had the waterpark to ourselves. There is a huge toddler area and the medium sized slides appeal to kids up to about age 8, but empty into shallow water, so your child does not even need to knwo how to swim to go down them. This was the first park that our 6 year old actually felt confident going down the slides on her own (she is timid about the water). And there were a couple larger slides adjacent to this shallower pool that we adults could go down when we got bored with the smaller slides...all in the same compact area. Highly recommended.