So, you're comparimg punching a defenseless child who's at least only half your size to punching an adult who can have the means of defense? You know what, I'm done with this, you're going way too far in trying to justify jokes about beating up kids.
Your assumption that all adults have a means of defense is an incorrect one, actually. Many adults suffer from things such as brittle bone disease and you wouldn't be able to tell unless they have an especially severe case of it, for example. I recently found out a childhood friend of mine has it and I never would have known otherwise. There was a case when she was younger where she broke her leg from, what I was told, was a really bad accident, but she recently told me it was just from what would be considered a minor, harmless fall off a step for most people. But for her, that was a trip to the hospital to fix her leg. If you punched someone with her condition even moderately hard, you'd potentially break a bone. Similarly, they probably couldn't hit you very hard because it would damage their own bones.
Assuming that all kids can't defend themselves is also incorrect as there are many who can. A violent kid is scarier than a violent adult, after all. Because children lack moral maturity, and an understanding of the sensitivity of different parts of the human body (and those who do understand can feign ignorance). I myself had to look after some pretty violent kids in the past. And while I'd never punch one, I've seen first hand the kind of surprising strength even a kid in the single digits can have.
So in closing, punching people IRL is bad unless it's out of self defense, regardless of age. You never know how much damage your best hook could do to a particular person, as you don't know how durable that individual is (unless it's like, your best friend and your having a bit of rough play). I'm sure many would agree to that.
But acting as if there is something especially wrong with making a joke about punching a CARTOON kid, compared to a cartoon adult, is kind of farcical. At the end of the day, they're both cartoons. They're not real.
And it's not like Nino doesn't get burnt to a crisp and skewered by electric blades from Hoshidan princes screaming "YOU DIE, NOW!" on the regular in FE Heroes. Yet it's punching her is where your suspension of disbelief ends?
Again, not defending the joke itself really, it's not one I would make myself. I'm the kind of person who actually doesn't like seeing my favourite fictional characters getting beat up and such, or having rude jokes made at their expense. But again, likening Pumpkaboo to a real child abuser over a dumb joke about a fictional character who can MORE than defend herself, is a pretty crappy thing to do.