Encourage the boys and girls to invite their grandparent's to attend Sunday school with them.
Prizes can be presented to the oldest grandparent; youngest grandparent; grandparent traveling the most miles to be present; and grandparent with most grand-children present.
The children could do a pictorial interpretation of "Grandparents" on overhead transparencies to be shown with background music on this special Sunday. Perhaps some of the grandparents and grandchildren could present special music. A grandparent with an interesting life story could be invited to speak.