Do what you can to make amends without hurting them by bringing their attention to what you did. If you bad-mouthed them to someone, for instance, tell that person you should not have been so ungrateful as to say something like that about your parent(s). You don’t want to leave the impression out there that you meant whatever it was that you did or said.
Keep in mind that if they have found out they may have chosen to ignore the situation. Leave that option open.