I would too. Definitely. If they join ISIS they're already forever lost to you and could hurt so many innocents.
Also, in theory, if a parent knows a child is radicalized and doesn't tell the authorities, and the child proceeds to blow up a night club or shoot everyone in a movie theater, the parents should be considered accessories before the fact. They knew something deadly could happen, that their child had murder on his/her mind but did nothing about it.