A baby literally born without a brain. It twitches and spasms like a zombie.
Have you seen that article about Myah Walker and her zombaby?
The natural desire for babies to put everything in their mouths will occasionally result in them looking as though they are eating the head of their brother, or the cat. This gives the appearance of being a zombie, or zombaby.
Aww, look at all those cute little kids, oh no, wait, look at that, is that a cat's head in his mouth? Is he eating that cat's head? Ew, Zo, zom, zombaby!
One zombaby would be fine, but having two zombabies would harm the parents. "Nnnot enougghh heaaadddsss!"