If your children’s clothes are still in excellent condition then I would highly recommend selling them to the Children’s Market. If accepted they offer much more per piece than Kid 2 Kid and Once Upon a Child. They do only accept well known brands that are in immaculate condition, which means you’ll be taking a trip back to Goodwill to drop off what wasn’t accepted.
I’ve been able to find great prices on boutique dresses for my nieces here also
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