Well, this is terrible to think about and I hope to God it doesn't come to pass, but it IS their account so if God forbid something happens to them before we use the 529, it would be part of their estate.
Not necessarily. It isn't an asset in the way a 401K plan would be. There is a person listed as the beneficiary of the account (the grandchild, for example) and the account is for that child but the grandparents are the owners of the account. If the grandparents die before the account is used up, the money sits there for the child under some kind of trustee unless there are contingent owners. Grandma and Grandpa should list contingent owners if they haven't already so the ownership then passes to someone else (the parents of the child or some other adult) when they die if the account hasn't been used up yet. It is a simple piece of paper that takes care of this.