Having just been there 2 weeks ago, the entire Dells area is really a pretty good bargain. Lots of the resorts come with "free" tickets to a water park. We stayed at the Flamingo...got a room for $65/night. And that included 2 tickets/day to Noah's Ark outdoor water park. Can't beat that!!! The boat rides/duck rides are something that kids LOVE (my son is 6 and he loved both trips), but I would suggest checking into your hotel first, or at the very least, picking up some brochures from the hotel lobby, because there are coupons galore in there (good for 25% off boat/duck rides).
For the summer, I prefer an outdoor waterpark. The indoor ones are good for Fall/Winter/Spring.
The Wisconsin Dells website is really good, and you can sort by bargains, etc.
Good luck and HAVE FUN!!!