I think your kids would be too young for Science Center, and the City Museum.
I'd stick with the Magic House. Our kids loved it when young. I've also heard a lot of great things about Grant's Farm. It's on our "to-do" list next time. I've heard great things about the zoo, but we haven't been there. It's not really "free" when they charge a lot for parking, and many other items once you get in. Omaha Zoo is closer to us, so that's our top choice for zoos. (Anymore, you need a bullet-proof vest to get to and into the KC Zoo.)
I'd stick with the Magic House. Our kids loved it when young. I've also heard a lot of great things about Grant's Farm. It's on our "to-do" list next time. I've heard great things about the zoo, but we haven't been there. It's not really "free" when they charge a lot for parking, and many other items once you get in. Omaha Zoo is closer to us, so that's our top choice for zoos. (Anymore, you need a bullet-proof vest to get to and into the KC Zoo.)