Not Immortal
In most cases grandchildren are crazy about their grandparents. They love them and they want to spend a lot of time together with them, at least when they are kids but not teenagers. When children grow elder they have different interest but they still love their grandparents, though they never spend much time with them when they are teenagers.
One way or another but love you grandchildren is never gone and the death of grandparents is a great strike for them. it is a great strike for everyone, though adult people are able to realize what had happened and they cry, and it hurts for them but they can live with it because they know that it is common.
Though, things are different when talking about children. When talking about kid who know nothing about death and about mortality of people and so on and so forth. It is important to calm a kid down. There is no need in talking about life and death and the other world. Try to explain your kid that grandmother or grandfather had fly away to the other better world.
Never say them that he or she will be back because they'll be waiting. Tell them they will see their grandparents one day when they will go to visit them. It is likely to be the best way for you to explain your child what had happened.