Perennial is one of my favorite Not For Profits in St. Louis. They teach sustainable furniture design classes among many other things. Clothing swaps are just the tip of the ice burg.
Refresh is a fantastic thrift store in Brentwood. Their clothing swap is organized by my friend Cristina Cousins. You can sign up for the swaps on the St. Louis Clothing Meetup Site here. You can also sign up for the kids clothing swap, The Moppet Swap on the St. Louis site as well.
I take any clothes that I cannot swap to my local H&M. They take everything! Kid's clothes, fabric scraps, stained baby onesies (no joke). Plus, they also give you 15% off an clothing item for using their recycling program.
Schoola is a new way to trade in gently worn clothes to raise money for your child's school. I am just learning about this program and seeing how I can get my daughter's pre-school to be a participant. Please check it out here.