Grandparent alive, son alive and grandchildren. If the grandparent dies without making a will then the son will have a legal entitlement, if there is a will giving nothing this is allowed as parents do not have to make provision for their children, but can be contested by the son. And that's costly to the estate.
If the son were dead before the grandparent, and there was no will, the grandchildren get their fathers legal right share.
The fact the son is a bad parent is not relevant. I don't think I've ever heard of a case of grandchildren disputing a grandparents will.