They won't accept the fact that teenagers are also capable of babysitting a child, because they haven't experienced anything different than grandmothers and, in rarest of cases -- grandfathers, taking care of their grandchildren. It's pretty much taken for granted. I guess it has something to do with social and family structure. It's not that common anymore for all family members to live and have a home together, but they still have more or less the same relationship to one another. So, most people think there's nothing wrong with leaving your child to your parents for a while, as if they lived with you in a room just across the hall, although they actually don't.