Yes, of course. And that is because the parents have a legal DUTY to protect their children. I'm not familiar with the legal duty of priests to report crimes. Last I heard there were things such as "spousal privilege", "priest-penitent privilege" and "doctor-patient privilege".
Perhaps there are some people within the FLDS that have a DUTY to others, but it isn't necessarily the case that ALL of the adults have a duty to protect ALL of the children.
Do you have a legal duty to protect your neighbor's children? Do you have a legal duty to protect the children of people who attend the same church you attend?
If you believe that your neighbor is smoking pot, do you have a legal duty to report them? If you fail to do so, does that make you an accomplice to their crime?
Well with the FLDS its not a neighborhood its one big family :-)
Physicians, other health care workers and teachers are some people that have a duty to report *suspected* child abuse in Texas.