He says she is homesick.
A perfect early example of one is the 1966 film of Bob Dylan's "Subterranean Homesick Blues".
A Wubbzy backpack is the perfect tool for carrying a little one's bedtime essentials to granny's house, and a Wubbzy cuddle pillow and blanket will soothe any homesick feelings at bedtime.
You can tell your child that there will be activities to keep him busy so that he doesn't have a lot of time to think about being homesick.
You can rest assured that it's impossible to become bored, and chances are they won't have time to get homesick.