I would guess it is too early to tell the sex of them. I am just now figuring out mine are males by their voices and they are going on 8 weeks. They say they have their adult voices at 10 weeks. Good luck on figuring out what yours are. They are beautiful ducks.
If it quacks it is a girl and if it has a more hoarse voice and is quieter it is a male. My girls quack very loud. Usually when they are picked up. It is a beautiful duck.
The body looks just like my Mallard hens but the head doesn't match. It might be a mixed duck unless someone knows of another duck that looks just like this one.