The FLDS are breaking the law. No religion can be protected in its illegal beliefs if they become illegal acts. So either change the law or desist.
None of what I have said below should be construed to mean that the FLDS have some protected right to practice polygamy. It is illegal and the perpetrators of that should be prosecuted. But to take their children away on the basis of a popularly held view that the religion's teachings are abusive is crossing that line that protects freedom of expression and religious belief. On that head, the Texas CPS may feel justified in taking all the children by calling the influence upon them "abuse", but then the can of worms opened would include any and all odious religious doctrines that millions of parents teach their children: all of that would be equally vulnerable to the same attack: remove the children ("protect the children") from the abuse of indoctrination.