You may also want to encourage them, gently, to participate in Catholic programs for youth that are sound, demanding, and fun. It may be there that they first come in contact with those other mentors and new friends who will introduce them more concretely to the possibility of a life of total dedication. These are just a few ideas. You yourselves will have others. Nobody knows your children better than you do or loves them more save God himself.
Vocations are a supply -side phenomenon.
Supply creates demand. Don't forget the shortcut of entrusting them to Mary, the Mother of God. If our Lady takes a special liking to them, her Son will form them into the new evangelizers of the third millennium. You will lift a hymn of thanksgiving to God for rewarding your generosity by calling one of yours to be a special one of His. Don't worry, your grandchildren will.