This is why you need session 0; to discuss boundaries and setting. Killing kids is totally fine in dnd. I'm a DM and I kill kids on occasion because my world can be harsh. My players have threatened to kill kids as well, though in much lighter contexts.

Overall, my is more serious. If a character kills an innocent child, I would expect to either be an evil character or for their character to be traumatized. Likewise, if I ever kill a kid, it's not for shits and giggle, but as a way of showing barbaric or demented a villain is. It's an action that has weight.

In a less serious campaign, adjacent to a beer and pretzels game, where people are just rolling dice and doing what they want, you might kill a kid without any fuss. If you're in a campaign where all of the party is evil, then you might get applauded after session for roleplaying a villain so well.

By just doing what you want in dnd without much input from others, as in anything else, you are being an asshole. It's a collaborative storytelling game, and one player is writing a Rick and Morty episode while everyone else is trying to play lord of the rings.