As indicated in my initial post on the Newtown horror, those who die young have a special mission in the spiritual world which can be accomplished only by those who have died young. So, to claim that they are Angels, which is a class of beings one step higher than humans, glosses over this important distinction, and tends to dismiss the efforts of the parents who brought them into the world, and who might feel that their efforts were for nothing.
Before we reincarnate, we have a general idea of what our impending life on Earth will include. So, it is possible that these children had agreed to take on this mission even before they were born. This would fall into the category of destiny, which shapes the general outline of our lives while leaving us sufficient freedom to exercise our free will.