A typical way children seem to get involved in cults is when one of the parents gets involved. This sometimes follows in divorce. The cultic parent then introduces the child to the cult during access rights.
Also, when both parents are in a cult, what do you do for the child's sake?
The thread below illustrates this case and it also includes other threads on the area of children in cults - some is specific to Landmark Education.
I like to keep all the threads on children in cults together for easy reference.
I don't have an answer to your legal questions but depending on what country you live in, a visit to your local member of parliament may be able to help if you have a specific case in mind.