The downtown hotel with the waterpark is the Depot. We have had some of our kids birthdays there. Good spot, not far from the dome, easy $5 cab ride for the family. Might have dome shuttle.
Here is the link. http://www.thedepotminneapolis.com
Both the depot and MOA hotel with waterpark are a little spendy, especially on weekends with the Twins in town. But still worth it!
Otherwise, there’s a Marriott property downtown with an outdoor pool, the Best Western downtown may have a pool and is extremely close to the dome. The Hyatt and Millenium downtown have pools and are on Nicollet Mall. You might be able to priceline something downtown last minute.
Just some ideas for ya!
ET