Oh gosh.
I live in bathing suit haven.
Here, everyone wears whatever for the beach.
Some wear bikinis. Some wear one piece suits.
Some or MANY wear (because of the hot sun and sunburn) a "rash guard" top, and then boardshorts with it. Both girls and boys and, adults.
"Bikinis" comes in MANY styles.
It is not all always, skimpy and tiny.
Even Grandmas here, wear bikinis.
It is not "string" bikinis."
Some bikinis are very conservative.
Take her to Macy's.
They have many bathing suits in the girls department. At least they do here where I live.
Per any type of clothing for kids, it is up to you. Then, each child has their own tastes.
Then you teaching her about dressing and about whatever you think is appropriate or not.
I've done that with my daughter since she was 3 years old. She is 10 now and has good ability to discern clothing. For whatever reason.
The thing is: at some point and age, a girl will probably get and wear, a "bikini" or 2-piece or whatever you want to call it. Even as a Teen. Or as a college girl or adult, etc.
So, at some point, you need to teach your daughter about clothing. And about choices. So that, SHE can also discern clothing and "attitudes" and her own style. So she is not just doing what other girls are doing.