I don’t know about funny, but it would be infinitely scarier if the kid weren’t a zombie. The cuter the baby, the deeper the horror.
Likewise, you’re not really invested once your head’s cut off. I’d make the victim fully conscious of what’s happening to them.
…which brings up the question of motivation. Why not simply overpower the child? Nobody wants their brains eaten.
Three answers:
A) They’re injured or tied up or otherwise incapacitated.
B) The kid’s got hypnotic powers. (and really, if you give a kid hypnotic powers before they grow out of the “claw at things and put everything in their mouth” phase, that’s sort of inevitable. they’re practically zombies to begin with.)
C) The victim is a willing participant. Proud and strong and trying to do right by their obviously disturbed child.
…okay, that’s all I got. Time to seek therapy!