Is it easier to get from Midway or O'hare to the American Girl store?

All my little girl wants for her 8th birthday is to fly to Chicago and go to the American Girl store. Since price is pretty comparable flying into O'Hare and Midway, I am trying to figure out which is easiest. I would prefer not to take a cab because of the expense, so what would be the best way to get from either airport to the American Girl store and back to the airport? We would be flying in around 9am on Aug. 22nd and flying back out about 8pm that same evening. Please keep in mind I will have my daughter and my mother who uses a walker. Any suggestions?