ThredUp is a site where you can exchange boxes of clothes that your kids have outgrown with other families.
Here is an excerpt from their site:
ThredUP is where Americaâ€™s busiest parents exchange boxes of clothes that no longer fit their kids for stylish ones that will. We coordinate swaps among a group of your closest friends or connect you with thousands of families nationwide. Thereâ€™s no bidding, no auction lots, no rifling through consignment racks, and absolutely no trips to the post office. All swaps are done online, with scheduled home pick-up and delivery. ThredUP is the next generation of swapping â€“ a kids clothing revolution!
Right now when you sign up you will get your first box for FREE! After, that youâ€™ll only pay shipping for each box of clothing that you swap!