But the fact that a child can be killed for a nonexistent identity clearly disproves that assertion.
Not really, no. This is along the same lines as the argument, "In order to deny the existence of god, atheists have to believe in him first." That is, they both ignore reality, as well as even the most basic grasp of logic.
When a person is murdered, it is not the person's beliefs, identity, appearance, whatever that causes the murder. It might be the justification for the murder, but it is not the cause. The cause, of course, is the murderer. The murderer, in order to justify the action to him/herself, creates a reason. The victim is the wrong color, the wrong religion, likes jazz music, whatever.
The reason need not even be a real one. If you kill someone because you suspect they a part of an alien plot, it doesn't automatically mean that they were part of an alien plot-it simply means that you believed they were, and that that was a valid reason to commit murder.
The same applies here. If a person kills a child because they believe that child is part of the wrong religion, it doesn't mean that the child is part of that religion, it simply means that the murderer believes they are, and that that is a valid reason for killing them.