Take one apart and redesign it to fit the smaller ones. Just like you do for humans. I think it's called making a sloper, a basic fitting shell.
It's been a long time since I made any Barbie clothes, but I made them from measuring the Barbie doll itself, just like I did for the kids' clothes.
I find it silly to pay just as much for a doll's dress as I did for the real kids' dresses. Sure wish I had stock in those companies!!