Hi L.,
I know what you mean, I have 4 kids of my own and need to find inexpensive ways to provide nice clothes for them. I go to a children's used clothing sale at Southwest Chicago Christian School in Oak Lawn. It's on Central Avenue. They have it twice a year, one in the fall and one in the spring. The one for this fall is on October 3 from 9am-12pm. There are no children allowed and you can't bring in any bags.
I also just recently found out about one in Lemont on Bell Road at a Nazarene Church. That one is next Saturday September 12 from 9am-12pm. On their website, they say that you can bring bags or laundry baskets, but no children.
These sales, along with clothes, have toys, books, puzzles, etc.
Let me know if you need exact addresses. I noticed from your profile that you live in Oak Forest, so these places are not too far from you.