There's a difference between sexuality and sexual content. Sexuality is how you identify your sexual orientation, whether it's straight, gay, bisexual, etc. and means a lot more than just sexual content. I don't think they meant at all that you were making it sexual, and I understand where they're coming from.
Kids are, to put it bluntly, stupid. It's a romantic storytelling trope of sorts to have kids be deep and wise - as a sort of unexpected contrast to what they're usually like - but realistically, kids are more concerned about what cereal their parents are buying for them than what sexual orientation they are. And even when kids think they're being insightful and wise, it's usually just them spouting stuff that most kids go off about ("world peace!") that they just don't understand the depth to yet because they just don't know (whether that existential time comes in elementary school or high school or much later on when they're in college, whenever); and of course, we as adults just nod and smile and think back to when we were like that.
I think that's all they were commenting on. They weren't saying that you were sexualizing your characters and making it a sexual concept, just that realistically, most, if not all kids, just aren't naturally deciding or realizing what their sexualities/identities are (and I say naturally because parental influence does come into play). It's a touchy subject that's more than just black and white, but you have to decide how you're going to approach it. Again, it has nothing to do with sexual content - but it has everything to do how you want your child characters to behave, whether it's like average kids that are always getting into weird shit for the sake of curiosity (I'm talking weird shit like sticking their finger as far up their nose as they can go until they're bleeding lol) or the kind of character trope where a kid is wise and insightful and aware of the world around them and how it works.
