When grandparents visit their grandchildren--whether it's for a day or weekend--the line on discipline is clear: The parents are in charge. Grammie and Grumps keep their mouths shut. If it's not a safety issue, it goes directly to the parents. And when grandchildren stay with you while their parents vacation, the line is also clear: Grammie and Grumps Rule.
But when the grandkid and your grown child visit you in your house for an extended period--or move in for economic or family reasons--where does the line of discipline land? After all, they're living in your house for more than an overnight. You have certain rules (no bathroom noises at the table, no bouncing balls in the living room) and if the parent is distracted or out doing whatever it is they came to your house to do [go to a conference; find a new job] are you the Disciplinarian In Residence?. Do you want to be?
It's a tricky issue. If your grown child is there for an extended period of time--say a few months--then it makes sense to sit down and clarify the disciplinary issues--what the rules are; who's the enforcer. But if it's only for a week or two, is a meeting and clarification necessary? And who knows in advance what issues are going to come up? If a child doesn't want to go to school or camp in the morning, whose job is it to work that out? It's not an easy call since the issue tends to rise like a sudden thunderstorm, and there's no time to coordinate responses--just time to run for cover.