I spent a lot of my time on the beach in my pants as a child. Lots of photos too and there’s nothing sexual about it.
If a girl wants to wear a bikini then let them!
In the blazing sun I’d put mine in a rash vest and bottoms but if the weather isn’t too hot then I have no issue with my daughter wearing a bikini. I wear one and she wants to be like me. She’s got no more to cover up than my DS. No one would care less if he was running about in small swimming trunks.