If people really think a young child in a bikini 'sexualises' them, then I seriously wouldn't want them anywhere near children!!!
I mean, i rarely see genuinely inappropriate clothing in real life but I definitely have on Instagram.
I don't think it's sexualising to point out that kids don't need to wear adult clothing (e.g. a short, figure hugging body con dress), worn by adults to look appealing. It's purely for the parents satisfaction to see a mini me.
(Not really talking about tankinis her, again, normal). I guess than can do what they like, but people saying 'that's not a kids outfit' is not sexualising.