robin, I just don't know what I'd do in your situation since it doesn't seem as clear cut as ConcernedMum's.
How about seeing the daughter and mother more often than you do now, getting closer to them to determine if there is a problem?
If you see the daughter on her own - ie on a playdate and she seems upset, could you gently say if she ever wants to talk anything over she can come to you?
Is the mother definitely withholding all food for 8 hours as a punishment? or would she let her daughter have some bread and butter, say? It seems a long time to me especially if this is a regular happening.