Kids and Scissors Don't Mix

Well, it has happened!  The kids' rite of passage - cutting their own hair.  Our neighbor's kids were over here last night and the girls were playing in the family room, while the boys were coloring at the counter.  Bella and her friend often come up and down the stairs and get toys, so I didn't think anything of it when she came upstairs and then went back down.  A few minutes later, she came up and said, "Mya cut her hair." - and it was hacked all over!  I wanted to crawl under a rock somewhere!  I was humiliated that this could happen on my watch.  We took her home and I explained it to her dad, who was gracious about it, thankfully.  I made Bella go to her room and lay on her bed for a 1/2 hour while "The Electric Company", one of the kids favorite shows was on, because I couldn't think of a more creative punishment.  She laid there and then we talked about why what she did was wrong and she should never get scissors without permission and that she should NEVER cut her own or someone else's hair.  Then she apologized and I told her that I forgave her and that we didn't need to talk about it anymore.  Then at dinner, I noticed that there was a section of her hair that looked choppy and, with further inspection, I realized that it had definitely been cut too.  But at that point, there was nothing more to do about it and it was far less noticeable than her little friend's hair!  Then this morning, I got an email from the mom of the victim that said the hairdresser that fixed Mya's hair said she couldn't have cut some of the pieces herself.  So I asked Bella if she had cut some of it herself and she said, "You told me that we didn't have to talk about it anymore after I was punished yesterday."  I have to admit, that even after Mya piped up and admitted that she had cut her hair herself, I wondered if Bella had cut it too, even though she said she didn't.  I think I just maybe didn't want to believe that Bella would do that, especially after we have told them REPEATEDLY to never cut their hair or to never get scissors without permission.  And since we have never had a hair cutting incident, I just hoped that they knew better.  Now I am mortified.  Not only has a child cut their hair "on my watch", but my daughter lied about her participation in it.  Now I am not sure if I should punish her again or if she has been punished enough.  I don't want to drag the situation out and brow-beat her, but I also don't want her to think that lying is okay.  But it is my fault also for not pursuing the truth more or not admitting to myself my own suspicions.  If I would have asked Bella more about it, she probably would have told me the complete truth.  But I was preoccupied with my own thoughts and things I needed to do and the fact that her parents were going to be upset either way.  I feel like a big failure as a mom right now because I know that I didn't handle the situation well.  Why is there not a book that says, "When ___________ happens, do ____________".   I am just thankful that they didn't stab each other or poke one of their eyes out with the scissors.  So, I guess it could be worse, because hair grows back.