Zhik, Ronstan, and DaKineall make XS Womens harnesses. They are usually on the bargain table or you have to order them.

The problem I found is the size chart for harnesses only goes down to 56 inches tall and 80 lb. You can make them fit a smaller person if you fabricate a smaller spreader bar. I have done that on the beach using an old style "wide" harness hook as a spreader bar.

All that said, the best I have seen for a kid was a scaled down version of a Hobie Lace-up. I have also seen a second set of grommets installed on the adult version to make them fit kids