And the problem with phantasmal killer is that it's not 3d6, it's save or die and take 3d6 anyway on a save. (Well, technically, it's save or save or die; Will disbelief then Fort partial or die. But it's DC 18 on both; by 12th level your good saves will be about +10, so a roughly 50/50 chance of failing each.)
Unless people realise they should kill the creepy child, it can pick off people with near impunity. How can you even tell it's the Child doing it? It's an SLA, no components required.
But then, I suppose that the Child only uses this SLA when its life is threatened.
By the way, is the Child immune to fear, or can you turn its phantasmal killer back on it?