Christian apologetics regarding the killing of innocent children during the Genesis flood I'm trying to catalog the various justifications that are put forth when it is claimed that killing innocent children in the Genesis flood was some combination of immoral, cruel, or unjust.

Here are some of the common answers I've come across:


God can do whatever he wants. (video video)
The children were not innocent, due to original sin. (link)
Society was extremely wicked during this period. Children endured physical abuse, sexual abuse, abandonment, starvation via neglect, child slavery, etc. The wicked men were killed out of justice. The children were killed to put them out of their misery.
The flood is allegory. God didn't actually kill all of humanity except for Noah and his family. (link)


Note that 3 is in conflict with 2.

4 makes 2 and 3 moot.

Proponents of 1 consider the rest of the justifications unnecessary.

What are some other justifications that people have put forth? Views from all denominations welcome.



UPDATE


Man was warned by Noah for 100 years before the flood came. When innocent children died, it was the fault of their parents for not building boats, not the fault of God. (See comment below by NigelJ and link)
Infants and children immediately went to heaven when they died in the flood so this is not ultimately viewed as cruel.
There is another way to look at this question: There is nothing different about the death of people who died in the flood and people who have died throughout history in the millenia since. 

The maximum age at death recorded in the Bible is 969 years, for Methuselah. To Noah, God revealed that he would henceforth limit the maximum lifespan for humans to 120 years. This is an eightfold reduction in the maximum lifespan. Since we do not see a terribly abrupt cutoff in the distribution of people's longevity today, we can assume that God did not just add a cutoff, he shifted the whole curve.

Today, 25% of people in the world are 14 years old or less. This was higher in the past, but childhood mortality has been declining due to better nutrition and medicine. However, in Noah's day, it is likely that childhood mortality was similarly low, compared to conditions after the flood.

If we apply an eighfold shift to the curve, then that means that in Noah's day, only 3% of people were aged 14 or less. That means that the flood of Noah killed fewer infants and young children than is the common situation throughout of most history since then. Stillbirth, infant diseases and starvation, child sacrifice, child abuse, war, weather, and all the other causes of death to children in the centuries since Noah probably have killed a higher percent of children than the proportion killed during the flood. 

This reduces the question: "How can you justify causing the death of children in the flood of Noah?" to another question: "How can you justify God's taking of the life of any child at any time throughout history?"

In Luke 13, Jesus said this:


  13 Now there were some present at that time who told Jesus about the
  Galileans whose blood Pilate had mixed with their sacrifices. 2 Jesus
  answered, “Do you think that these Galileans were worse sinners than
  all the other Galileans because they suffered this way? 3 I tell you,
  no! But unless you repent, you too will all perish. 4 Or those
  eighteen who died when the tower in Siloam fell on them—do you think
  they were more guilty than all the others living in Jerusalem? 5 I
  tell you, no! But unless you repent, you too will all perish.”


Jesus did not teach that the circumstances of a person's death correlate simply to how good or wicked that person's life was. The book of Job teaches the same thing. Job's friends confidently asserted that Job's children were exceptionally wicked because of the storm that killed them. They said the same thing about Job, by comparing his financial and physical condition to what they expected should happen to wicked people under God's judgment. God rebuked them strongly.

I know - I haven't answered the question. But the question as stated assumes a category of children that we do not need to deal with any differently than the children alive today. For that, Elihu's answer to Job in Job 37 is appropriate:


  11 He loads the clouds with moisture;
  he scatters his lightning through them. 12 At his direction they swirl around
  over the face of the whole earth
  to do whatever he commands them. 13 He brings the clouds to punish people, or to water his earth and show his love.
  14 “Listen to this, Job;
  stop and consider God’s wonders."


Sometimes suffering is punishment, and sometimes it is training that produces growth and is evidence of God's love.

As for what happens to those children, as a Reformed / Baptist believer, I believe that God welcomes such children into heaven.