It is always just for God to do as he wishes with his creation.

Naturally. The concept of 'just' becomes meaningless when applied in this way, just as the concept of 'good' loses its meaning when anything God might do is always good.

That's a very negative view you have of children. I personally think that we are inclined towards both. Finding out would be a matter of empirical study... there's probably some psychology research somewhere which looks into this.