I agree that the Depot is likely to appeal mostly to little ones. Not surprisingly, it has a train motif. They will organize birthday parties in the waterpark, which is normally open only on weekends. The Renaissance Hotel is nice, with modern rooms plus some unique suites in the historic part of the building. The attached Residence Inn is an older one with dated kitchens (but flat screen TVs). The buildings are connected by the waterpark and share facilities such as the (fee) garage/parking lot. You can order RI room service from the Renaissance menu, but it's delivered in paper and plastic rather than nice dishes.

Near MSP the airport Radisson has a waterpark with big slide. It's close to MSP airport and the Mall of America, with shuttle service. There's a second hotel a bit farther from MSP, IIRC an extended stay property (perhaps Hilton family) or a regular hotel and an extended stay hotel, that's attached to a public waterpark, I believe at a local public park. It has a fee for parking (in the suburbs).

The Wisconsin Dells (rental car needed, this family resort area is several hours from MSP, and probably the same from Milwaukee MKE, but closer to Madison MSN off the interstate) have several hotels with big waterparks. Great Wolf Lodge looks like the most impressive one of those that can be seen from the road.

There's a waterpark in a hotel on the outskirts of Duluth. If you follow Superior Street (one of the main downtown streets) north-east out of town toward the North Shore, this hotel is on the right. I don't think it's a major brand and I forget the name, but it doesn't look all that attractive from the street. Duluth airport is served at least by DL with IIRC mainline and RJ service. It's about three hours from MSP by car, all along interstate roads but you must go north to the other side of Minneapolis/StPaul from the MSP airport.

Thee's a big and well known waterpark in Belgium.