It is a lot like parents forcing their children to sit on Santa's lap because it is about the parents getting what they want, not their child having their needs met.
What you might do is let the parent know that it is obvious that the child doesn't want to have their face painted now and that it might be good to come back later as it is very difficult to paint on someone who doesn't want it. Or you could say that you have worked with a lot of children and it really is your philosophy not to force yourself on a child who doesn't want their face painted.
I think we are at a time when parents must walk the razor's edge and be an advocate for their child at the appropriate times and at other times, give up what they want and allow their children to do what they want and to have a say when it is appropriate.
All parents should take heed at what C. is telling us. Where is it that we are this selfish with our children and where is it that we should change our way of thinking about how we parent.
Thank you for this, C..