Magpies from Ideal will be pretty, but most likely they WON'T look like the pictures you see of magpies. They'll have way more color, and in wrong places. I haven't personally ordered from Ideal, but all they have up on their site are drawings of magpies, no photos of their actual birds. I've seen photos of hatchery birds, and they almost never look anything like those drawings of magpies. So personally, I'd skip the magpies in favor of another breed. On the other hand, if you want them to just be colored and don't care much whether they look like the pictures of magpies on their site, they are a relatively calm, slow duck.
I'd skip runners if you want them to be calm. Runners are more energetic and agile than other ducks, and as a result they tend to seem more flighty. Khaki campbells, in my experience, are also less docile than other breeds.
Welsh harlequins are probably an excellent choice, and maybe buffs though I have no personal experience with them. How about a "hatchery choice" crested duck? Crests are adorable, and you could just let yourself be surprised by what kind of duck you end up with.
Good luck with whatever, I'm sure they'll be adorable.