There is a good chance the parents did not notice the damage, they may not have really looked at it. I would bring it up to them in a polite gentle way. It may have been the girl, a friend of the girl's or the parents themselves. For now you suspect the girl as being irrresponsible and it may not be the case. I would not ask for them to pay for the camera at this point.
As far as others stating one should not lend out stuff, that is terrible advise; a good friend of mine is a member of this forum and I borrow more from him than he does from me.
Some one mentioned not lending money to family members. My brother was terrible at paying back and often would not pay back until you asked him for it. One day I needed some money and borrowed $20 from him. About 3 hours later I returned and gave him the twenty back and his response was "What is this for" and I told him I borrowed it earlier in the day. He shrugged and said that if I said so it must be true. He simply did not remember lending and I think he also did not remember most of the times he borrowed.