Kids make terrible zombies: running around trying to bite adults kneecaps and balls. The episodes with kid walkers would be too comedic and would not fit in with the rest of the show,
It really was dark as hell....seeing a child hit that level. Realistically though - that kid was likely a schizo in a normal world, so to be in a world where you can just shoot fucked up people like that - probably helpful.
Seems it's become even more popular over time. Even the encore airing displaces the shows that used to run at 11pm