Ok, Don't laugh. Try hissing at them or have your nephew and Mom spread their arms to their sides like an airplane when they approach. That's what my geese do to show whose boss when they are threatened. Also you could catch them and hold them for your Mom and nephew to interact with them and while the geese get used to them. You could also have them bring a treat and feed them each time they come. They start warming up really fast then.
I usually have guests feed them through the fence first. That usually makes both the guests and geese happy.