In a letter to a granddaughter, a grandmother should include one or more statements of love, the date, and openings and closings familiar to her granddaughter. The bulk of the letter should consist of whatever the grandmother wants to share with her granddaughter.
Statements of love show the granddaughter that her grandmother loves her and is thinking of her. The date is important so the daughter can keep the letter and always know when her grandmother wrote it.
The grandmother should use openings and closings familiar to her granddaughter so that the letter sounds natural. If she and her granddaughter have nicknames for each other, she can use her granddaughter's nickname in the opening and her own nickname in the closing.