It all depends on your version of christianity and the time you live in. First of all: Angels are a separate order of being form men. Children may become saints, but not truly angels. The RC church has many child martyrs (The Holy Innocents for a start) but these have saintly status, and are not cherubs.
If you are a catholic, then an unbaptised child will go to purgatry after death: original sin causes a stain. This is harsh, but untill fairly recent centuries it was believe they went straight to hell, so the sentence has been reduced a bit.
Protestants tend not to credit original sin, but do not believe in saints either, or not so much, so heaven is the best you are likely to get.
Buddhists and hindu's would say that a child dying young is fortunaate in that it will be reincarnated closer to perfection next time, as it cannot have built up much sin.
Im sorry to go on, but there are many myths surrounding the traumatic death of the young. Every belief system has a variant. The Problem of Evil, to which you allude, is a major theological stumbling block: If God made the world, and is good, why is there evil?
Not an easy one. Becomes easier if you accept the non-existence of god, though.