Reality: Children die every day of the year just as do other humans. There is no promise that a child can escape reality because he/she is innocent of willful sin.
Hardly.......do not confuse "justice" and "deserved" punishment with murder. The children you speak of were being raised into a paganistic society (that worshiped the false gods among the Midianites and others)......these children were being taught that it was OK to steal children from other tribes and burn them at the stake as a human sacrifice to appease their false gods. Ask yourself this question? No child can be found guilty of sin into he/she reaches the age of accountability to reason right from wrong, freely choosing with comprehension and understanding that which is evil......is it better to be called by God while there is no requirement to be pay for a sinful life than to grow into such a society and pay with your enteral soul?
God dealt out justice and declared that such a seed among men must be wiped from the face of the earth to avoid more ritual sacrifices at the burning alter. These people were guilty of stealing the children of the Hebrews to use them as sex slaves or worse as human sacrifices. During the O.T times the Hebrews were bound by a law, An eye for an eye and a tooth for a tooth.
God even went so far as to punish the King with the loss of his crown because God instructed him to destroy everything to include the livestock....etc., He keep back of the spoils of war and was rightly punished for disobeying God.
As far as what you call innocent there are several instances where Children were killed along with the evil people that parented them. As stated, Children are not held accountable for the sins of their fathers and they are to be with God upon death. No child is held accountable (Ez. 18:20) for their parents willful sinning.
Among the children God ordered put to death as a righteous and deserved punishment, The Flood (Genesis 7), Death of the first born (Ex. 12:29-30) and the total destruction of the Midianites (Numbers 31). All because they would one day grow into willful sin themselves if God did not take action and punish their parents "righteously". They were being punished with the same measure they were found guilty of.
Inversely.......atheists have no problem with killing children (abortion on demand)......often stating that its better to kill the unborn than bring the child into a world were no wanted to feed or house them. Some logic no? Preemptive morality?

There is no morality in nature........thus no atheist can claim that morality has evolved from nature. Morality is transcendent to mankind in its logical origins.
Regardless, as stated previously........the O.T. is required reading by everyone that would attempt to please God, to learn from the historical mistakes made by those under the O.T. law and the historical prophecies that direct us to the truth that Jesus is the Messiah of prophecy.