With that age this time of year, I'd vote for the Mall of America hands down. I'm not sure how long it's been since you were there but they have added so much in the last few years. Crayola Crayon Experience, Sealife aquarium, American girl store, plus Nickelodeon theme park, mini golf, Rainforest Cafe. There is a Marriott on property so you wouldn't have to step foot outside. You can also take the light rail all over the cities (Minneapolis and St Paul both). There is also the Children's museum and the Science museum. The Minnesota zoo can be done mostly indoors as well, the way it is set up. From Chicago it's only a 6-7 hr drive so it's very doable! (We live near the cities and drive to Toledo several times a year, so I'm familiar with the drive!)