I will be driving with my 2 daughters, ages 9 and 13, from Minneapolis to Milwaukee around August 20.

Are there any interesting places/activities on this route?

What do you mean by interesting? The short answer is, "no," but at least it is a fast road. There are things like the Circus World Museum in Baraboo that the girls might like. Madison is nice and somewhat scenic. Of course you pass right by the Dells if you wanted to hit a water park or something. I have made this drive many times, and really usually just try to get it over with fast.

A more scenic but slower way is to take US 61 from the Twin Cities along the Mississippi, to 90 at LaCrosse, which rejoins 94 at Tomah to continue to Milwaukee.

You don't say how much time you have....If you can dawdle, then you could drive straight across Wisconsin towards Green Bay and head up the Door County Peninsula for a day or two, or more. (It'd be about five hours driving time). Wonderful beaches, several state parks, very family-oriented resort area.

From mid-Door County to Milwaukee via I43 is a three-hour drive.

I would second the suggestion of taking the short detour along the Mississippi River to LaCrosse.
Not far off the freeway near Baraboo and Merrimac is Devil's Lake State Park--good hiking, swimming, and rock climbing. You can take a ferry across the Wisconsin River and rejoin the freeway later.
Wisconsin Dells is only an option if you want go-karts, water parks and endless junk. But your kids may love it.
Madison is a nice city (Go Badgers!) The Capitol Square, State Street, and Campus area are nice.

Madison is a must, particularly if you can be there on a Saturday morning. The farmer's market on the Capitol Square is not to be missed.

Madison of course, just south of Devil's lake is the Merrimac Ferry, the only Free Car Ferry in the State (maybe the US)just a short trip across the Wisconsin River, but neat.

If you and your daughers are shoppers there is the New Johnson Creek Outlet Mall closer to Milwaukee.