I don't know what it is with daughters and mothers, but I already see these little struggles starting with my seven year old -- me wanting her to cut her hair, her not wanting to, me caring desperately if she doesn't keep her hair brushed, hating it if she wears something I think looks bad. But my own struggles with my mother were SO BAD I doubt I will ever forget them, and it is from the standpoint of the daughter in this that I write -- let your daughter figure out what she wants to do about starting to develop -- if you push her, unfortunately you are unlikely to get what you want and it will not help her feel good about these changes because she doesn't own them - you do! In my case I wanted to wear a bra when I probably had nothing to justify wearing one, and my mother was against it, wouldn't let me. Then when I was older there were some clothes I loved that didn't allow for a bra, and I got into top-of-my-voice arguments with my mother about that, and wound up iestablishing my values with a new guy I was seeing on a first date by taking off a bra I'd been forced to put on under my favorite shirt (it looked bizarre and totally inappropriate, in his car, the second we were out of the driveway. Not, I am sure, the kind of thing you want to be going on. Let her find her way -- bodies changing is weird enough without suddenly having parents acting angry and worried just because you don't want things to be changing yet. Since she's athletic, I bet it won't be long until she disocvers the practical benefits of the sports bra. But if you make it a bone of contention, not only will it take longer and possibly lead to kinds of out-of-sight resistance you do NOT want to be encouraging, but it will generally give her confused messages about who her body belongs to, or at least it did me. One I was out of my mother's house I went braless for years, sonetimes very inappropriately, until I finally figured out why *I* needed to be wearing a bra -- that it was something that protected ME from unwanted attention and kept ME comfortable. But I strongly believe that if my mother had just supported me, shared her own policies (when did she start wearing a bra, why where one, what does she do with clothes that don't allow for a bra, etc.) in a non-intense way, and let me find my own way, I'd have been wearing a bra proudly since I was 12. Her turning it into a struggle over who owned my body and sexuality left me in doubt about that into my 20s, and dressing inappropriately to PROVE my body was mine.
Hopefully your daughter won't be the bundle of hurt, defensive willfulness that I was in my teens and twenties, but I figure it's worth knowing how badly things can go when a mother decides that its her way or the highway. Your daughter sounds great, and she's agreed to start in the summer, which is in a month. Wait, bite you tongue, focus on everything that is right about her, including her body, and when you do give her a couple sports bras once the summer has started, try to be loving and positive. Fine, of course, to set limits about her not swimming in kiddy clothes when she's becoming a young woman, but again, avoid the drama. Say as little as possible, and assume compliance -- when she wants to go swimming in the summer, since you've already made the boundary, lovingly give her her new, simple tank swim suit and say "I'll go change and meet you in the living room!" Try not to make it a battle, try not to make the swim suit or the bra represent defeat. It's so important for our daughters NOT to see growing into womanhood as a loss of power, of having to conform to new forms of submisison, a narrowing of their alternatives.
All best wishes!