Several people I've come to know thru a support group were not legally adopted by cults, but placed in the care of cult leaders by their parents. One group is an offshoot of Opus Dei, and the child at 13 was shipped off to South America for better schooling. The other cult was a home for problem children, whose leader somehow managed to convince the parents their kids were alcoholics. These children remained in the home for most of their pre-teen and teenage years. Their care was administered by cult leaders and they lived away from their parents.
Found the link to the Opus Dei offshoot.