Please please get at least 2. Ducks can get very lonely. Also if you get them as ducklings, they thrive much better with another or others than they do alone. Not saying you’re doomed or wrong if you only get one. We raised a single duckling and added him to our flock with no troubles. But definitely are much happier when they have a companion.
We have a very diverse flock, a drake and hen welsh harlequin. Their population was very scarce and they are getting integrated back in so awesome choice in my opinion! You should always get a drake and hen and take a shot at hatching/selling ducklings. By the sons of your 4year old she would love that process and being able to see it all first hand.
We also have a Cayuga hen and I agree with your daughter they are so much fun to watch molt and develop snow feathers.
I would suggest ordering from hatchery then you can select breed and have them be sexed for sure. Depending on the hatchery will depend on the minimum they will send. A lot of them will also hit you with a small order charge if you have less than 10 selected. Ultimately it is cheaper to order more than less so finding someone near you to split an order with night be ideal.
If you went to local stores usually their minimum is 6 birds, in my state of NY anyhow. No where around us ever had welsh harlequins not stores or local farms. The stores do typically have Cayugas though, but it is all straight run. If they are a few days old you could vent sex them but that’s only if they let you which some places won’t as you can harm the duckling if done wrong. If they don’t let you or the bird is more than a few days old then you’ll have to keep them until you can sex by voice.
Hope you find what you’re looking for. Such fun animals to have, in my free time I’m always reading more and expanding my knowledge because they are so interesting.