Time can go by cruelly fast. Before you can say "amen hallelujah" the children are grownups, ready to "be fruitful and increase" (Gen.1:28)!
By then the matured parents look at life differently, many times with remorse: "O how I wish I spent more time with my children, but it was work and business and work and busyness... But then God send a second chance - a grandchild. Don't spoil them, but bless them with quality time. So if you see this old man playing marbles with his grandchildren, don't think he's lost his marbles. He's just grabbing his second chance.