The responsibility lies with the parents. What are they doing sending a child so young alone? United is not a baby-sitting service.
They are, the parents very specifically opted into (and paid for) United's $99 "baby-sitting" (unaccompanied minor) service. Let me quote from the parents's letter:
> The United personnel who gave her the wristband and instructions told her very loud and clear to “only go with someone with a United badge on and that she would be accompanied at all times”.
I don't think this even needs additional emphasizing, it stands on its own.
How's it not? As you noted United charge for an unaccompanied minor service[0], the parents believed everything was squared and United completely dropped the ball. Had the service not existed they could have opted to not send the child to camp, try to get what to do drilled into her and sent her on her own or found somebody to accompany. Then they could have been at fault if things had gone wrong.
But here? I fail to see how any fault can be put on the parents. We can debate between the fault being 100% United or 80% United and 20% bad luck, but the parents? Not in the presentation of the article.
[0] And the complaint letter adds details:
> The United personnel who gave her the wristband and instructions told her very loud and clear to “only go with someone with a United badge on and that she would be accompanied at all times”.
Even so, you're right. It should be like 99% United, 1% parents.
Besides, unaccompanied flights are awesome as a child. I did that once, don't remember how old I was. Flew with the German Lufthansa from my parents to my grandparents. You can't imagine how adult I felt, flying (almost) all alone. I remember the attendants were all friendly and helpful, and the gentleman next to me even swapped his seat with me so I could look out of the window. It was a valuable and memorable experience.