My tween daughter (just about to turn 10) is a major tomboy. Boy clothes, boy colors, boy interests - inasmuch as those things can be stereotyped. Also, as it turns out, she wears boy's boxer-brief underwear. She has always hated girl underwear because she claimed it gave her wedgies. She wore her training pants underwear for years after being potty trained, because it still fit (she's very slim) and it was the only underwear that she found comfortable - until we tried boy's boxer-briefs. Girl's "boy-shorts" did not cut it.
Personally, I think it's great that she has found something she's comfortable in. The problem is that she's at that age where what other people think really matters - so, while she is comfortable in boy's underwear, she is mortally afraid that one of her friends, babysitters, or some random stranger will find out. The major drawback to this is that it means she does not want change into a swimsuit in front of her peers (though she has no trouble being seen in her boy's swim attire), and therefore does not want to be in any camp that involves swimming - despite the fact that she loves to be in the pool.
I'd really like for her to just "own it" and be secure in the knowledge that she is wearing the most comfortable undergarment for her, and if anyone tries to tease her about it, just let them know that she's more comfortable and that it looks cool too. But that ain't happening.
Maybe she has the right idea. I know how cruel kids can be. But is there any way around this obstacle, that will allow her the comfort of her underwear without the fear that someone else will find out?