A frum therapist would be a start. The older they are and the long they have been OTD the harder it is to get them back.
As a parent all you can do is love them unconditionally and always leave the door open for them to come back. On the other hand make sure that they don’t bring down the other kids in the family. Don’t sacrifice the other kids who are on track for the sake of not alienating the OTD kids.