While it may sound unfair to punish parents if their children sext, children and teens should not be allowed as much alone time with technology as they typically are. Parents should be helicopters when it comes to their children and technology, in order to protect them. If a child is sexting, it means that the parents have not set enough boundaries with what can be done, or talked to their kids enough about what they should and should not do in regards to technology. Punishing the parents would send a strong message to them to be more diligent with their kids.
What would be the benefit? Parents can't stop kids from sexting without forbidding them to own any electronics, forbidding them to borrow electronics, and forbidding them from using public libraries and internet cafes. Altogether that would be more damaging to the kids than sexting. For that matter, what IS the damage?
There's no benefit.