This raven costume for girls includes the dress and headpiece. You'd want to outfit your little one further with black shoes and black stockings.
If your child doesn't want to wear a full on raven costume, they could accessorize a simple black dress or black shirt and pants with this raven headband.
For boys, here's a Raven Reaper costume. I'm not sure if this is based on a video game or cartoon character or not? Anyway, it looks like a raven!