K.M.
Kansas City has a nice zoo, if I remember correctly. It has been several years since I've been there. Also, the Country Club Plaza is a nice place to walk, shop, eat and people watch (it's just called "the plaza" by the natives there). Kansas City is also known for their fountains. It's worth the time to find them and enjoy them. There are several on the plaza.
Another thought is a place called Kaleidoscope (sp), part of Hallmark cards. I know they used to have a place where kids can imagine and create things...sort of arts and crafts like. Kansas City Northern Railroad has or used to have good things for kids who like trains, including a short train ride for not much $$...less than a dollar, it seems like.
There are some nice parks for picnicing and playing. There's likely one near wherever you choose to stay. PM me closer to time when you know where you'll be staying and I'll send you some other info.