It's because the children are the next generation of slaves. The older generations die off and become less useful. Demographics are destiny. Control the children and you control the direction of the society.

Hitler said something once about this and he's right.

When an opponent declares, "I will not come over to your side," I calmly say, "Your child belongs to us already...What are you? You will pass on. Your descendants, however, now stand in the new camp. In a short time they will know nothing else but this new community." -Adolf Hitler

This is why they target children. They believe if they indoctrinate the children then your dissent is meaningless over a long period of time.

Sex specifically is an easy means of controlling the young.