For the girl, how about Nanako Dojima from Persona 4? The only thing is that her hair is brown, not black. But the costume itself is comprised of very simple colours.
For the boy, I can't recall anything different from what others have mentioned. But I'm going to push on the two previous points about chibitalia or Sayoran. But the latter might be the more expensive one to pull off.
Anyway, good luck with your cosplay!