I think you could do more to vary the skin tones of the characters, they're all pretty pale. The values on the characters, values meaning the range in the darkness and lightness of the colors, should vary more within each character. They're coming off as pretty flat as is.
While I recognize that simplicity is the goal with this set of characters, I still think they lack a lot detail. Maybe seams on the clothing, maybe some frills on the blonde girl's dress, maybe a bracelet or necklace for one of the characters, dirty hands or a bandage, a hat or cloak of sorts. Anything that sets characters even more apart and gives a clue into their personality and upbringing.
This is a personal gripe, but I really don't like the way the eyes are stylized. I've never been fond of the eyes over the hair thing but if you like it, then keep it. I also think the eyes could be made just slightly smaller so that the faces indicate forehead space. And also consider showing hairlines as well. And lastly you put a lot of effort into hiding the ears of all the characters. I think you should have a least one, maybe two, human characters showing their ears.