I'm sure my mom has joked that she's going to "smack my butt" while laughing and joking. lol.
I guess I don't see using the phrase "I'm going to beat you" as anything to joke about. I don't understand how that could even be funny, even if you were laughing while saying it.
I just think it's inappropriate.
I think it's one thing if all parties involved know it's a joke... but sometimes jokes can cross a line so quickly. I remember being a little kid playing catch with a ball with my dad. He threw it to me and I didn't catch it and it whacked me in the face. We laughed about it. I threw it back to him, and he threw it back to me - and this time intentionally hit me in the face with it. Like seriously hit me in the face, it hurt bad enough to make me cry.
This was over 20 years ago and this memory still bothers me.
I don't believe kids need to be handled with satin gloves or anything but I do think people should be careful. And honestly, in this day and age, saying stuff like that is asking for trouble, even in the privacy of your own home... God forbid the kid goes to school and says to another kid in play, "I'm going to beat you to an inch of your life!" and the teacher asks where the kid heard that, and the kid says "Oh Mommy says it all the time!"
So yeah... I don't know... it's good if everybody involved knows it's a joke and is laughing about it but I do feel it's inappropriate and could escalate quickly.