I have always wondered whether there are any cultural or political reasons for the fact that the Donald Duck cartoons depict an unmarried man (well, male duck) with three nephews. Perhaps the artists did not want Donald to have a wife, and having three sons without being married would have been considered immoral? Do we know anything about the original considerations of the artists? Are there any contemporary sources commenting on how unusual this arrangement would be?
Great question! I am not an official historian, but am a filmmaker (officially) and an animation nerd who has too much time on the internet.
Al Taliaferro was the creator of the triplet nephews, and was the artist for the Silly Symphony comic strip. They were introduced on October 17, 1937, which predated their first animated short, Donald's Nephews, by six months. Originally, his twin sister Della has them visit him for the day. As seen by other spin off series, namely Duck Tales and Quack Pack his guardianship became permanent after a time. Donald was their guardian. When he joined the Navy, the nephews got shuffled over to Uncle Scrooge. Della didn't even make an appearance in Disney media (except for as a child in The Life and Time of Scrooge McDuck) until the 2017 reboot of Duck Tales, which is a pretty darn welcome addition to the Duck clan, in my opinion.
As far as I can tell from researching this, there was no 'higher' motivation for the addition of the three rascals. They were simply added to cause misfortune and mayhem to an already testy Disney character. On the note of their first short, how does Donald get 'Modern Child Training'? From the chaos caused by the brothers. It's a common gag in the old and even modern ones, that a character will discover their ‘Book Ex Machina', per se, due to some event happening in the episode. That doesn't necessarily rule out that there could have been Donald offspring planned; Daisy was a character after all. and Donald was always, ahem, enthusiastic to see her. Of course, in Donald's Diary, he may not be suited for marriage. That can leave the question of children for him up in the air, especially considering the time period it was made.
Shoot, even the second lesson of the book is 'Never Lose Your Temper in Front of Children'. That is the quintessential problem for Donald. He is outburst incarnate. His temper is always his downfall. He cannot control them, despite his best attempts. They always have control, something Donald rarely possesses. Worst of all, they are kids.
On a side note, the nephews’ father has never officially appeared in any media from Disney. In the debut comic, Della mentions in her letter that the boys had sent him to the hospital from a firecracker under his chair. His name is never mentioned. Almost seemed like there was never supposed to be a brother-in-law for Donald. From all the shenanigans they pull, obviously they are not easy to deal with, possibly providing a motivation for sending them to Donald, who is closer to their temperament than the parents.
(Full disclosure, from this point on, is more of a subjective POV. I am pulling from what I have noticed from watching countless hours of old and new Disney cartoons)
Therein lies the importance of Donald to his nephews. He is a father figure to him, albeit a massively flawed one. He's around. He takes care of them. He loves them, even after some of the hijinks they put him through. Even during his enlistment, he sees the boys, and secured a guardian for them while he was gone. Quack Pack, to me, is a great example of how even though he is 'Uncle Donald', he's their dad. He tries his darnest. Even if it can be a low bar.
As much as he helps the nephews, they help him. They straighten each other out as they go along life, whether is for Donald to lose control or for the boy to exert a little more over what they do.
Edit: there was a lot I wanted to include about Al Taliaferro, but it was not pertinent to the question. He was definitely an interesting person, but the important thing was that he was the creator of the nephews.