I competed for years... And my 10 has been on the rec-league for 7 (he lives gymnastics but competition just isn't his thing, so no pushing :)
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Gymnastics leotards are usually made of Lycra / or another heavier duty stretch fabric (like some of the velvet ones) are a LOT stronger than dance leotards (so they don't rip off you while tumbling... Gymnastics is a very 'contact' sport, and also so they don't stretch out).
That SAID... A few times with a dance leotard probably won't be disastrous (depends on how thin it is).
There ARE solid color gymnastics leotards. ESP solid reds and blues (because of the Olympic aspect of the sport, there's a lot of USA colors... Solids, flags, stars & stripes), in addition to the more swimsuit style wild and crazy patterns.
Plain colored is FINE (unless she's on a team that has a uniform pattern). But you REALLY want to get the sturdier leotards.
Hotshorts / spanks are 50/50 (some love, some hate... Some gyms allow, some don't). They're more common now, so you're less apt to be told to take them off, but the do need to be skin tight so they're not catching on the equipment (boys and girls use different equipment).
Underwear is a BIG NO-NO.
This is the one that new parents (and dads!) tend to good on. No no no no no no underwear!!!! Especially if you're concerned about modesty, because it means that everyone will be staring at Bunche lumpy privates, and due to how active one is, the normal body movements ALWAYS start shoving it through leg holes (and then not only are people staring, but your daughter will be griping and digging at herself a LOT). Big no-no. :)
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Saftey Side:
No
- Jewelry (ESP earrings which get ripped out, but also rings/ watches/ bracelets) which can cause injury when they get caught
- floaty clothes (shirts, skirts, loose shorts, sweats) which can also cause injury getting caught AND block the instructor from seeing if their form is correct (form is only 2% about being pretty, and 98% about avoiding injury)
- Hair up (so it doesn't get ripped out, fall in your face blinding you when your arms are busy and you need to see... As well as so it doesn't get caught in equipment causing neck injuries)
- No tights or socks (you need traction that bare skin offers!!!)
I know you didn't ask about those, but since skirts and sweats were brought up in other posts, I just wanted to put up why those are NOT good options, and why most good Gyms don't allow them for saftey reasons).