Gather all of your child's clothes in one place, such as his bed or the middle of the floor. To achieve a full clear-out, you need all items of clothing in view to sort them.
Get four large bags or boxes and label them "donate," "sell," "throw away" and "recover." Arrange them around the pile of clothing you need to sort.
Sort through the clothes, gathering all items that no longer fit your child, clothes he doesn't wear anymore and clothes in any condition that no longer meets your standards. Put all other clothes back in the closet and drawers.
Bag or box the unwanted clothes. Place clothing in decent condition in the "donate" bag. Put clothing that is expensive or in great condition in the "sell" bag. Clothes that are too stained or damaged go in the "throw away" bag, and items you can recover with a little mending go in the "recover" bag.
Get rid of the boxes or bags immediately. Don't hang onto them or you're not really purging them. Take the "donate" bag to a charity or thrift store, throw away the useless items and begin the process of selling the items in the bag labeled "sell" by posting ads online or setting up a garage sale.