Apologies for missing the question !
I have two thoughts on this (apart from naughty Donald obvs !).
1. Permanent separation where everyone can still see each other until the ducks become bigger.
2. Behaviour aversion therapy - we do this with a nice long bamboo cane. It means spending time with them but with a minimal presence. As soon as theres duck behaviour you dont want, put the cane down between naughty duck and their intended victim. No need to touch anyone with it or have it whip through the air - just let them know the stick turns up and blocks their path. Ducks are super clever and work it out very quickly (a bit like learning to leg it when the net comes out

).
Donald probably wants some, erm, one on one time with his ladies and sees the duckling preventing that so is a bit grumpy.
Good luck !