In my opinion....I would stay the extra days in South Dakota and go to Cuter State Park instead of the Dells (since you are not doing the waterparks). We stayed at Beaver Lake campground with our kids....and we all had a blast! There is so much to do and see without spending a lot of money...safe hikes and swimming with a view that is priceles....Bonus, go for a ride with the kids and see the herds of Buffalo and feed the Donkeys!
I live in WI and have gone to WI Dells as well as Devil's Lake many times. Devil's Lake is beautiful!! and there are some great hikes and swimming right there..but I agree with above....get a campsite quickly! : ) WI Dells is very comercialized and everything is pricey. Although there is beautiful landscape it easily gets missed by the "tourist trap" attractions. One more note: If you do want to do a waterpark for a little cheaper, stay at the Flamingo Motel, you get a free ticket to Noah's Ark Water Park, which is fun...and if you know someone in WI, they can buy you cheaper tickets through their Credit Unions/Banks.