I had no problem with it, but I know people do/would/could. However, I could not think of one good reason why he shouldn't dress up like Ariel, or any girl. Because girls are members of an inferior gender, something to avoid being mistaken for at all costs? Because when a boy wants to wear girls' clothing, he should feel ashamed and hide away those horrible feelings? The 'best' reason I could put into words: "Because of what people will think." And that felt so wrong to say, I didn't even consider saying it. Why should people's opinions about something like that change what you do? Should whispers in the hall cause you to drop out of Mathletes? Should you throw away that sweater your grandma knitted because a 'friend' thinks it's stupid, even though you love it? He wasn't masquerading as a girl for any evil reason. He wasn't trying to trick anyone. He wasn't even trying to make a statement on society. He just wanted to be Ariel because he loved "the way she swims and because she collects human things."
So, we put together an awesome (if I do say so myself) costume. It had springy-sproingy red hair, a seashell shirt, and mermaidish sandals under his almost-tail. (It was an almost-tail so he could walk – I did not want to carry him the whole time.)
I knew most people would simply assume he was a girl. Side note: People also say, "She's so cute" about him when he's wearing sports clothes, camouflage, or even going shirtless in the summer. So, as we were making the costume, we talked. He and I decided if people said something about him being a girl, he would just go with it. He would be acting, staying in character.
People did think he was a girl, and he did just go with it. He shyly giggled when people called him Ariel and asked if he was in the show. It was a wonderful show, we had a great time, and we both got lots of compliments on his costume.
A few months later, we bought tickets to another show by the same company. Peter Pan. Kids were, once again, invited to wear costumes. I was prepared for Tinkerbell, especially since he has seen a lot of Disney's Tinkerbell/Fairies movies. I thought a Hook or crocodile costume would be fun to make. His favorite character? Peter. I named other characters, just to be sure. (Kids do have a tendency to change their minds a lot.) Yep, Peter Pan. So, we shopped for (more green!) supplies and hot-glue-gunned together a Peter Pan costume.
At the show, he shyly giggled when people called him Peter and asked if he was in the show. And, once again it was a wonderful show, we had a great time, and we both got lots of compliments on his costume. We looked at the cast photos on display in the lobby, and I read some of the actors' and character names for him. While a group of ladies was talking to him about his costume and the show, one of them said something about "her" costume. My son smiled and said, "I'm a boy." Then he said, "I know why they think I'm a girl. It's because Peter Pan in the show is played by a girl."
And there's nothing wrong with that, either.