Minecraft wiki entry for Mob_grinder, associated text: Lava grinders are typically ineffective against baby zombies, because even if they fall through a pool of lava, they often survive, and they can fit in the one-block space under the lava being suspended by the signs. It is a good idea to put a fence between the hopper and your collection room (a fence over a chest does not prevent the chest from opening). Then, if a baby zombie happens to be waiting on the other side, you can easily kill it.