Sexual? No, but violent. And I'm pretty sure 99.99% of DnD campaigns are violent in nature. So you basically have someone thinking "Hmm, I need a new character. Given that this is DnD, They'll be an adventurer who goes into dungeons or similar dangerous places, where they will be regularly confronted with violence and death of varying levels. Analogue to a Soldier going into an active war zone. Oooh, how about I make it literally a child?!"

It's the act of specifically choosing a child to be put into kill or be killed situations, when one has the option to choose anything else too, that's weird. There's kinda no real reason to do it. "I think it'd be interesting" is not a reason good enough to actively want to roleplay as a child who is forced to kill things, probably even other humanoids at one point or another, just to not be killed themselves. Wanting to play and put a child into that situation is just not appropriate.

When its underage players at an all underage table with all underage PCs, it's fine, I guess. One underage player at an adult table playing an underage character with other adult characters is weird too, but that's on the adults, not the underage PC.

But I'm pretty sure it's safe to say those are extreme minority cases anyway. At least in my experience, underage characters (mostly girls, really, barely ever saw an underage boy PC) are almost exclusively played by... those type of players. You know what I mean.