When I bought mine at WDW they did have children's and adult sizes at that time.
I bought an adult one but didn't try it on. It fits but they were/are made small. It can be adjusted smaller. I did contact Disney after I got home and actually wore it a few months later. I know my head isn't large as I have never had a problem with hats. I get a headache if I wear it for more than an hour or so.
If you do see children's sizes make sure it fits or try out an adult one and adjust it smaller.