J.M.
Try Second Chance on Armitage Avenue (at Sheffield), if you are in the city. The area is conjested and difficult to tolerate (full of trendy boutiques and sparse parking) but the quality of clothes and the selection is amazing. You might make an excursion to the Lincoln Park Zoo and then stop by afterwards. Also, there is a great family-owned ice cream shop one block away. There was a store called KidBiz in the South Loop, on Dearborn. I think she had to close for low volume of sales. She says she'll reopen in another South Loop locale sometime soon.
I often check out consignment shops in trendy, upscale neighborhoods. You can get some fantastic deals on some very nice barely used merchandise. I've found a Coach bag for $30 and Prada shoes for under $50 at a store called Selections on Clybourne, near Webster.
Childrens clothes hold up especially well.
Good luck.