M.L.
I am a hairstylist and a face painter (my sis is a professional clown) and many children this age will scream and cry if we attempt to paint the face.
My suggestion, is to buy a non-clogging powder in an ivory color and blush it on her cheeks. Or, do a small little ghost face paint on one cheek, but not the whole face. Not only that, but washing the paint off of a child's face can be difficult as well, or they will smear it all around as they eat candy or rub their eyes or whatever throughout the night.
The baby powder would work on her hair. Just style it, maybe put the bang section back in a little bow or headband so powder won't fall in her eyes, then sprinkle some baby powder on it. Just start out very small, and add a bit more as you go so you know how much her hair can take.