All parents hope their kids excel in everything they do, but the truth is--that isn't always the case. I recently wrote an article, targeted towards youth, on how to confront parents about low marks. In the article I emphasize the importance of being honest. I also state that parents will appreciate a mature and responsible approach from their children.
Read the article here.
Additionally, I'd like to say to the parents, try and be patient with your children. It's really easy to get angry with them over things like low grades, but if you yell or do not give them a chance to explain themselves, you cut off communication. The consequences of burning the bridge of communication far exceeds the consequences of bad grades. If your child does not feel safe or comfortable confiding in you they may start to feel isolated, without help or hope, and may turn to other individuals for guidance--people you may not want advising your kids.
Try and have discussions with your children. Allowing them to open up to you may help them gain the confidence and skills needed to avoid having to talk to you about poor report cards and lead to more conversations about their personal successes.
Best of luck!