She's ten and mostly plays with the boys, fitting in well due to her easy-going nature, outstanding athleticism, strong sense of fair play, and wild and crazy sense of humor. She's super competitive, one of the leaders of the pack in terms of ability, while still stopping to help the kids who need rules/strategies explained. The problem is, from her point of view there's nothing on the other side of puberty to look forward to. The boys will be getting stronger; she will be getting breasts. Not fair! She's miserable about this and it hasn't even started yet.
Since a large part of the community are also members of StackOverflow, I’ll try translating this question to mathematics.
Say you are ten and you are good at math. You're always looking things up on Khan Academy, you’re very competitive (Math Olympiad, anyone?), but you still help the non-math kids with their math homework. You and your math friends go to math camp, are learning to program, have Raspberry Pi on your Christmas lists.
Recently you found out that over the next four years, most of your friends’ math intelligence is going to get a four-fold boost. Even some of the kids you help with math will get this boost. You, on the other hand, will get a two-fold boost. Plus, you are going to have a French accent whenever you talk about math. Not an ugly French accent – it will be pleasant, but it will be noticeable.
So, how happy would you be if someone said to you, “That’s okay, you don’t have to be any less mathematical, just because everyone else is going to be better! It’s not only about being the best. While other kids are doing calculus or even multivariable calculus in a few years, you can still have fun with trigonometry! Or, why don’t you learn to cook? You get to use math to measure things in cooking, right? Besides, don’t a lot of cooks have a French accent?”
What can you say that is comforting but not patronizing?
EDIT, seven years later:
My daughter found the reality of being a girl wasn’t as bad as she thought it would be. She remains better than 98% of the boys at her chosen sport, and she finds it hard enough to compete with the girls at the National level that competing with the boys is no longer a goal she worries about.
TL;DR: Things continued fine until seventh grade. Seventh and eighth grade were not great. At lunch, she had always sat with the boys at the “baseball/soccer table,” but in seventh grade they started looking at pron on their phones during lunch. They didn’t exclude her, but she was uncomfortable, so she found some boring girls to sit with (“boring” because they weren’t interested in sports, but at least this group wasn’t obsessing about boys and clothes and make-up).
The school part of her middle school experience remained enjoyable, but things got worse in the sports area. Several of the boys got better than her at baseball, and the middle school baseball coach preferred giving playing time to the boys, even when they weren’t nearly as good as she, because “the boys are going to be the future of baseball at this school.” This naturally became a self-fulfilling prophecy.
She switched schools for ninth grade and decided to try dressing like a girl. Here’s where things got interesting, because in doing this, my competitive kid found a whole new area to be competitive in. She found she enjoyed dressing to look good. At least a third of the girls at the new school were as competitive at sports as she, so she made good female friends; the boys were more mature that those at her previous school, so she made good male friends. Breasts were not as problematic as she’d expected, partly because she’s so athletic that hers didn’t get too big.
So, it worked out. It’s still doesn't seem fair that guys can build muscle so much more easily than girls, but there are enough other things going on in her life that she really only notices that when she’s in the gym.