From a rather pro-nudism perspective, I would argue that a bikini might even be more provocative than toplessness or full nudity is, because it sort of singles out the chest as an area that deserves special attention, which seems rather inappropriate to me with prepubescent girls. I would definitely favor a one-piece bathing suit for younger girls like that from a modesty perspective, as it seems to offer the best of both worlds - keeps all the potential taboo areas covered while not singling out undeveloped body parts as taboo yet.
But irrespective of your culture's attitude to nudity, I think it is important to establish that different clothing styles are appropriate for different situations. And the stakes for that are probably even higher in cultures where nudity is not just a blanket taboo topic. There is a time for streetwear, a time for swimwear, a time for nudity and a time for formal attire, and while there is certainly a bit of overlap between those groups, there are some things that are inappropriate, be that getting naked at school or wearing swimwear to church, streetwear to the pool or formal wear to the beach.
