That' a very open question, JaneAnn. How old is the child, and do they have any religious faith?
I think a lot depends on how much they understand, and whether they think the grandparent is in heaven or just 'not here'.
My children were very little when my father died. My daughter was a baby, so too young to be told anything, and my son was under three. We are not a religious family. There was a rainbow in the sky a couple of days after he dies, and I told my son that Gramps hadn't had time to say goodbye, but he would be happy if whenever son saw a rainbow he would think of him. For a little while, he did comment when he saw a rainbow that it was Gramps smiling at him, but he soon forgot, as he was so young. It seemed to help him at the time though.
When my FIL died, they were old enough to understand - maybe 10 and 12 or thereabouts. They were upset, as they were old enough to have had a proper relationship with Grandad, and to understand what dying meant. They also saw that their own father was upset, which was difficult for them. There wasn't much we could say, apart from to let them talk about him whenever they wanted to, and to decide for themselves whether to come to the funeral with no pressure either way.
Death is very sad for those left behind, and faith must be very useful if you have it, but as we don't we acknowledged the sadness and tried hard to concentrate on remembering FIL happily. He was a good man, so that was easy in some ways, although obviously difficult in others.