I know this stretch of road so very well. My parents are originally from Minnesota and we lived in Wisconsin and now I live in Illinois.
Some of the comments assume taking 90 out west to get there (Janesville/Beloit). We live closer to 94 and take that north through Milwaukee.
Specific potential stops for you:
Madison - there is a mall very near the highway. I think it's East Towne Mall. I really, really don't like malls. But if the weather is bad, you can have a picnic in one, and you can get out to walk and have some nice bathrooms.
A unique place to stop is in Tomah at Burnstadt's. It's just a little off the highway where 90 and 94 split. It's has a small grocery store, some antiques, a clothing store and a restaurant. Again, I'm not a shopper (really!), but it's some place different, and you can stretch your legs and have a decent bathroom.
Someone else recommended Mauston and that is a good stop with different fast restaurants and gas.
There's the Norsky Nook that is in Osseo, Wisconsin. Oseo is a very small town, and the restaurant is downtown Osseo. You can get your lefse and a slice of pie here. Osseo also has a decent park if the weather is good (my family held a reunion in Osseo for many years).
I agree with the many rest stops along the way that are fine to stop at. I do not remember any of them having swings or play equipment with kids. But bring a frisbee along or a soccer ball for some play time if the weather is good.
Have a fun trip!