If your underwear doesn't have sizing tags, it's a very good idea to label them yourself. It makes it easier for you to know where to place it, in which child's drawer it belongs. I personally like to use puffed paint size them. It's great to use assorted colors. Every size a different color, so the children, even the little ones, choose their size by seeing their color. Puffed paint never wears out, it always lasts.
This sounds like a great idea. I don't have trouble with underwear, but I have always wanted something to mark socks. I have a terrible time matching socks with the person. I have tried permanent markers but they wash off in the laundry. I'm gonna try your method. Thanks for posting