CYP - my guess is that the abuse was covered up (which is an indication that the parents knew it was wrong and tried to concel their acts of violence on the kids) by the kids wearing jackets or long sleeves. Also, considering the kids were home school and not allowed to "associate" with any of the worldly neighbor kids, this kept them out of the eye of the public, thus, allowing the abuse to continue unnoticed.
I suppose it is like the family that let their daughter starve to death because it was against their religion to eat meat. She was not getting enough to sustain her and she was very frail and underweight. However, it do come out in court that the father would eat meat during the day when he was at work. When the kids testified as to getting in trouble if they ate things that were not allowed, it was heart wrenching. They would get spanked or punished because they were disobeying god. Guess what, the judicial system found the parents not guilty of murder. They were put on probation. WTF!!! How is that possible. Well here in the States, religion is still looked upon as a "personal" decision and if you hindge something on the "against my religion" it usually gets you off with a slap on the hand.