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What is really the cause of this abuse on children?
Mental illness, pretty much pedophilia: sexual attraction to children (both male and female). And a lack of control over those impulses, which ultimately lead those sick individuals to act on their urges and abuse children who have not even gone through puberty.
Ironically, most of people who suffer from those attractions and do not want to abuse children won't try to get psychological help due to the high stigma there is on society against pedophiles.
If they understand they have a problem and it is something abnormal, then they should be guided to get psychological and psychiatric assistance, so they won't abuse children in the future.
On the other hand, those who had already abused children should be either locked up or chemically castrated.
Many abusers were abused when they were children, so the cycle keeps going on. Usually, it's someone from the own family who perpetrates the abuses. Sometimes other family members know and remain in silence, allowing it to happen, and sometimes the story is only discovered later, when the child has already become an adult and starts speaking out about it.
There is an impression the number of cases are increasing these days, but I also suspect it's just harder to keep it in secret nowadays compared to past decades. Moreover, I believe one of the greatest reasons behind the chaos this world is living nowadays regards social interactions, increasement of hostility between people, inability to set genuine bonds, is related to abuses against children.
"but whoever causes one of these little ones who believe in me to sin, it would be better for him to have a great millstone fastened around his neck and to be drowned in the depth of the sea" - Matthew 18:6