I attended  a workshop given by Dr. Victor Cline a licensed clinical psychologist. He listed several keys to raising successful children. I think grandparents can have a strong influence in many of these areas. I’m just sharing a few,

Set reasonable work standards backed by appropriate discipline.
Foster autonomy and independence.
Develop and strengthen children’s conscience and values.
Set reasonable limits for TV, video games, computer access.
Teach effective social skills.
Stimulate your children’s intellect.
Your can read more in his book How to Make Your Child a Winner.