Observation #2 "They said I look like a boy because I have short hair"

The after school program I work at has some diversity within it. There are some African American children, and some Hispanic children also. There are also some whose primary caregiver is their grandparent, or adopted parents. I thought I would more then likely deal with race and class issues before dealing with gender issues. I guess I was wrong. The other day a fourth grade girl with short hair came up to me. She looked extremely sad, so I asked her what was wrong. She said that two boys at her table had said, "girl that looks like a boy" when addressing her about something. She then began to cry. She said how people everywhere, even elderly people in grocery stores sometime thought she was a boy because of her short hair and how much it hurt her. I felt so bad for this little girl. I basically sat down with her and told her how girls can be beautiful and feminine with long or short hair. I told her how earlier that day I saw a women at my school who was bald, and how she was beautiful. I also told her how there were many girls out there that had short hair for many reasons. It's easier to care for, it doesn't get in the way, and during the summer it doesn't stick to the back of your neck. So there were a lot of pluses to having short hair. After I had made her feel better, I decided to go talk to the two boys that had said she looked like a boy. I brought them out of earshot of the other children and began by asked them if what the little girl said was true. They admitted to it, which was a start, but then began making excuses. They said how sometimes other people had called them girls before and that didn't bother them, that they thought it was funny. They thought what they had said was a joke. I explained to them how much they had hurt their peer, and they did feel bad, not to the extent I probably would have liked, but still nonetheless it was their idea to go and apologize to her. The reason I spoke out against the short hair incident, is because I didn't want the two boys to think that it wasn't okay for girls to have short hair. I wanted them to know that it was perfectly acceptable. I really didn't know how else to tell them what they did wasn't nice, and that girls can have short hair and not be mistaken for a boy. Does any one else have any ideas at what I should have said?
This event was significant in that it really opened my eyes to how much gender is still an issue in everyday life, even in educational systems. This event really was oppressing in that overall it involved boys telling girls they couldn't have short hair or else they looked like a boy. In their mind, girls had to have long hair. I didn't know fourth grade boys could be sexist? Being part of this event has made me more attuned to what is going on around me, including peoples side conversation. Yes, nosy, I know, lol. I just want to find out how every one thinks about gender. I pick up on responses people make in public about things as well. Sense that incident I have noticed how in a movie theater, a girl coming into one alone is more likely to sit next to another group of girls, then a single guy. Or if there is a seat in beside a girl and a guy, a girl will usually take the one next to the other girl.