You mention prizes. Is this a competition? Is she going to be entering this painting to be judged against others in her class? I mean, I agree with daisy, stand back and let her pick what she wants. I even think the suggestions for help and guidance here are a bit much. I don't think you should influence this at all. No suggestions. Let her lead the project. How old is she? It depends, I guess, on if she is 5 or 10 or 15. But if this is a competition, NO HELP. it isn't fair.
