If your child comes forward and reveals their wounds to you, it can be shocking and very distressing. The fact that they have so openly shown you what they have done to themselves may lead you to think that they have just done it to upset you or get your attention but it's more likely to be a cry for help. This again leads back to the reasons behind self harming.
If they talk to you about how they're feeling (which is a very positive step and one that all parents won't be lucky enough to see) but they can't pinpoint any one particular event or thing that's happened to make them do it, you might feel like they're just making a fuss or not be able to get your head around it. It's likely they just don't understand the motives themselves, let alone be able to explain them to you so try to have patience with them.
Often people that self harm will be embarrassed and ashamed of their problem and try to keep it a secret at all costs by hiding their scars under clothes or just pretending to be OK in public. It can be hard to know that anything is wrong and in this case, attention is probably the last thing they want.