When I was 6, my best friends (who happened to be girls) decided we should go and get a soda together, and the three of us dressed alike. They of course saw me wearing panties, and then my mom did.
She asked me if I liked them and I admitted I did. They were certainly prettier and felt better than the boys' undies I had been wearing. I told her I wanted more, so after that she started buying me panties.