I have to agree with St. Louis. It's an easy drive and there is TONS to do! The zoo is awesome and free the Magic House (which is a children's museum) is so great you could spend easily 2 full days there and of course there's everything else...arch, Ted Drewes frozen custard, museums, etc.
I've wanted to go to Omaha too and have heard they have a great children's museum as well as zoo, but I'd check the weather before you do that! It could either be super cold or just right in Omaha right now. Des Moines isn't that great in my opinion. I don't think there's enough to do really.
I've also heard Great Wolf Lodge is fun, we've never been though. There is also Coco Key by the stadiums which is lots of fun and my 2 year old LOVES the water park just as much as her older cousins.