Internally... I'd think it was hilarious. Being told to fuck off by a 10 year old who you raise and make food for everyday is low key hilarity. But to start I'd ask them why they said that. Why it's not an appropriate way to speak and how that can be hurtful and why cursing isn't something you don't do all the time. We'd move to punishment later. It's not that big a deal, children are emotional.