A message from a parent to his or her daughter and son-in-law should reflect the parent's personality and their wishes and dreams for the new marriage. There is no wrong or right way to write in a wedding card. Regardless of the message, if it is heartfelt and sincere, the daughter and her spouse will appreciate receiving such warm thoughts about their new marriage.
One idea is for the parent to write down the best marriage advice that he or she has gotten through the years. If there is a fond memory that he or she has of the couple, a written recollection of this event may be included. An inspirational quote concerning love and marriage is another appropriate item to incorporate.