Is it true that the the years we are alive are pre destined? is it also true that when a child is murdered his spirit goes straight to the highest of realms to sit by the foot of the throne of G_d?
There is a concept of “mazal” which the Talmud tells us determines the amount of years we will live. The Talmud does say, however, that through prayer one can “change” this “decree” and prolong the set number of years they were assigned.
Regarding the death of children, I do not know of a source that says that they sit in the “highest of realms,” but the Talmud does say that until 20 one is not punished in the Heavenly court for transgressions. So one receives reward for his or her mitzvos but does not get punished for sins.
Rabbi Moshe Rosenstein