You really have to wonder why anyone would do this. In a sick way I can understand molesting them, or murdering them because they were too much trouble and they wanted to enjoy life, but why would anyone who hated kids spend hours of their time and make their main business in life to adopt them, homeschool them, discipline them, and even torture them? It would be more fun to go out and play a game of golf.
About the only motivation I can guess at is some sort of sick religious fanaticism, wanting to bring kids up in a certain sort of way to serve some religious or cultist ideal. And I am NOT normally a critic of religion. But this sounds more like a Jim Jones sort of thing than anything else, at least until we see some other explanation.
You forgot sadism as a motivation.
Some people are just cruel SOBs who enjoy the suffering, pain and misery of others. They inflict suffering for the sheer joy of it.
It's totally alien to normal human beings, but sometimes a cigar is just a cigar, if you know what I mean.