Thanks for the info. My first thought was that it could be some modern TV series with Disney characters as kids; I didn't expect it to be from Quack Pack, since even though I have only seen a couple of episodes (not my cup of tea) I knew that series has an adult Donald. Of course there was nothing to prevent a flashback of his youth from being shown.
Anyway, I just finished watching many scenes of the episode and I see it's not a flashback, just Donald temporarily getting younger from a shrinking formula. Maybe it's better this way, at least they didn't screw up the story of his infancy.
So, I guess the only pieces of animation to show kid Donald are This Is Your Life Donald Duck (1960) with its "hatching from an egg" scene, and the weird short Donald's Better Self (1938), which works on the premise of Donald being a kid despite the fact that he is drawn as an adult.
This post has actually made me really curious about Donald's youth (he's my favorite of the "primary" Disney characters). I may have seen the cartoons you mentioned above but it would have been years ago. Going to look for them now. Thanks!