Yes, WI is unpredictable in March.
Just wanted to chime in that if you're looking to explore the area, about an hour south of the Dells is Spring Green. There you'll find 2 interesting places to go - Taliesin, the original home and studio of Frank Lloyd Wright. Not very (young) kid friendly, although DD did great as a 9 year old last spring. DH and I are both very into architecture though, and it's kinda rubbed off on her.
The other place in Spring Green is The House on the Rock. I don't really know how to describe it but eclectic. It's a weird, interesting, odd thing. This guy built this house on a rock (duh) but it was more of a party house. Over his lifetime, this guy collected things. Like, everything. Guns, books, lamps, ivory tusks, circus toy sets, doll houses, mannequins, carousels, pipe organs, replicas of crown jewels, Titanic memorabilia... so there are these buildings housing his collections of STUFF. There are coin operated machines tucked in. If you feed them one of their coins, they will play music.
I can't even begin to do that place justice....
http://www.thehouseontherock.com/index.htm
We stayed at the Inn when we were there. Their pool was AWESOME, but their water smelled gross - actually, the pool water smelled like pool, but their shower water smelled funny. I think it was well water, and has much more sulfur or something than we're used to.
We spent an entire day at the attraction and we were dead tired at the end of it. We couldn't stop looking at all the random stuff. It was creepy but interesting. I can't explain our fascination. The whole place weirded us out, but we couldn't look away - we HAD to see it all. I mean really - the House on the Rock - no bedrooms, every room had a lounge-y bed, and everything was covered in carpet. What kind of parties were they hosting there???