I don't find this funny at all.
Brat slaps cat across the face kind of hard. Animal abuse sucks. Not funny.
But the cat goes straight for the face and launches the kid off the bed. Brats getting comeuppance. Funny.
And before anyone asks;
video. Looks like the cat was being kinda rough to begin with, but cats don't go into that ears-back/pupils-wide/fluffed tail twitching attack mode for no reason, so I'm guessing the kid fucked with it. Also, cats meow like that when they're scared, it literally means "Leave me alone". Looking at just the GIF, you might think she's hissing, but it's really a desperate-sounding mew.
Also, parents are laughing at their kid getting hurt, so they're idiots, which means the kid is probably an idiot. Idiots don't deserve pets. Cat wins by default.