The Real Reason for Nephews/Nieces Than Sons/Daughters
The real reason why we often see nephews/nieces in the Duck and Mouse universe and other toon universes rather than sons and daughters is because introducing a nephew or niece doesn’t violate the continuity of the stories in a given work as much as introducing a son or daughter. It isn’t usually a matter of censorship as many people say. It's just that when you have umpteen stories in a work that establish that a character has no children, you can't have a grown child appearing out of nowhere in the character's (umpteen+1)th story as if the child has always been there. You could do it (as in the case of Goof Troop, they did it), but it would generally violate everything about the established continuity, as well as possibly disappointing readers/viewers who feel their intelligence has been insulted. That's why giving a character a nephew/niece is often the only way to create stories in which an adult character, especially with those in cartoons and comics, interacts with a kid.
