I'm gonna say your ducks are probably welsh harlequin mixes. My duck Snowflake is a mallard/WH mix, and she had the similar head and tail spot like your ducks did. See below:
Below is her during her teen stage (3-4 weeks) when she started darkening up:
Below is her today at two months old. She has her adult plumage. I think she is just gorgeous. She has very soft markings around her eyes and white-tipped feathers on her wings. The only problem is, her name doesn't fit anymore. She's not such a snowflake, lol. But I sill love her.
Below is her mother, Jeepers (front left), who is full welsh harlequin. Her father is my full mallard drake, Potato, who is also in the photo at the very back. He is the father of all my mutt ducks, I believe, and is quite the little Casanova
The two yellow ducklings in the back of the picture below are Jeepers and my male WH as ducklings. No black spots like Snowflake had.
So yeah, your ducks are likely to be WH mixes, or at least some kind of mixes since they don't really fit any one breed, but they're still so gorgeous. I just love mutt ducks
