Check out the National Child Care Information Center. nccic.org/index.html.
I found this:
Children Home Alone and Babysitter Age Guidelines
Most States do not have regulations or laws about when a child is considered old enough to care for him/herself or to care for other children. Currently Illinois and Maryland have laws addressing this topic. States may have guidelines or recommendations. These guidelines are most often distributed through child protective services and are administered at the county level.
Contact the National Clearinghouse on Child Abuse and Neglect Information (NCCANCH) at 800-394-3366, and staff there will refer you to your local child protective services agency to learn about age guidelines in your area.
The following organization and publications contain information about these topics:
State laws in Illinois and Maryland;
Examples of child supervision guidelines;
Examples of babysitter guidelines;
Demographic information about the number of children in self-care; and
Information about how to prepare children to stay home alone and to be babysitters.
A woman that I used to work with had DSS called on her for letting her 12 year old stay at home for an hour after getting off the bus. She was told that the age was 13, but that’s been years ago. I would say, do your homework. Make sure that your son and nephew are old enough for NC law, if there is NC law, and then make your own judgement. If your grandparents live next door, let them know that the boys are going to be staying at your house and they will have a list of chores to complete daily. You do not have to inform them that you have checked on the age guidelines with the State already.
I understand the frustration, they are your grandparents and you were probably taught like I was to respect your elders. It may not hurt to let them know that you appreciate all they have done for you while reminding them that you are the boys parents/guardian and this is the way you want things done.