I have many fond memories of the Dells having summered there my entire childhood. I can't even imagine navigating the Dells with kids with no car. Being a bit old school, the 'waterpark in your hotel' seem a bit over the top to me but considering the wide age range you are dealing with may be the easiest.
Older motels will still have pools and tend to have a better rate when you consider your two youngest won't really be able to take advantage of the waterparks anyway. It all depends on when you are going and the deals offered. An in between option is staying at a condo you can rent. Especially on Lake Delton there are loads of condos trying to rent out their spaces that never sold due the economy. Its like staying in a townhouse and you have a pool and play area but they are less intimidating than say Kalahari or Wolf, plus you are on the lake and beach!
As posted by others, the reason the Dells exist as a tourist destination are the amazing natural water formations. These are visible by Duck tour (a WW2 land/sea vehicle) or a cruise tour on different parts of the Wisconsin River and area lakes. If your 10 year old goes definitely check these out. Mini-golf and go-carts would be a hassle with the younger ones. Your family is probably the perfect age for Circus World museum in neighboring Baraboo. Little kids and big kids love it because there are shows through out the day to fit shorter attention spans and all the action is up close unlike say Tommy Bartlett's ski show.
While you are in Baraboo there is a little tiny local zoo and park that is free called Ochsner. It is along the river and perfect for a picnic and afternoon or morning of play and quiet. Old school park equipment, grassy hills to roll down and Prairie dogs and llamas to watch. North Freedom is another neighboring town with an excellent attraction for your families age, the railroad. Mid-Continent Railway museum is a living history museum so you can ride the rails as well as check out all the cool old cars. Depending on their intrests, the family might also enjoy the Stand Rock Indian Ceremonial, the night one is super cool by light of a bonfire.
If you go up to the Broadway strip in the downtown Dells shopping make sure to get some Monk's burgers and Dairyland Fudge! A lot of food near attractions is very sub par but these old standbys are yum. Pedro's is good for mexican food too.