And what has a small girl to be modest about?
A string bikini is about covering breasts.
A one piece is about covering breasts.
A tankini is about covering breasts.
There really is no difference in the function of any of them. Some cover up the stomach as well but their function is to keep the breasts covered. And the lack of breasts in a small girl is what people are claiming the "wrongness" stems from. If this were the case they would object to all but sun protection suits and trunks with or without a t shirt.
DD wears or does not wear the top half of her bikini as she pleases. It doesn't bother me either way.
A top padded to resemble breasts is sexualisation. Flat fabric is not.