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In your case, all it takes is for a wedding guest to trip over your bag, hit their head and end up in the hospital. The injured party may not want to sue you, but it is possible their insurance company will. And it is also possible they will not go after you, but after your parents."
This is where I believe the importance of not charging comes in. Feel free to correct me, but I think this is how it works. If she did decide against charging, she (or her parents) would only be liable for damages if anything like the above scenario were to happen. Also a judge would be more likely to take her age into account, and put the liability on the person who tripped for not being careful. Also, by not charging she is just a guest with a camera, which means the hall itself would probably cover the damages anyway. If she were to charge, that opens up a whole can of worms. If they sued, she (or her parents) could get in trouble for her operating a business with the proper liability insurance. Judges are generally much harsher with business operations and much more likely to give a tough ruling. Also, the bride and groom could get in trouble for conducting an illegal contract with a minor. The chances of these things happening is unlikely, but these things do happen.