My younger brother is 10 years younger then me and once at a movie, his friend was having a birthday party and I was there to see a movie with another friend (he was 8, I was 18) there were 6 8 year olds and two parents and the theatre still demanded that I be present with him at the movie because I was his guardian not the other kids parents.
I asked the theatre about the policy and found out that they had been extremely busy and since my movie ended more then 30 minutes after his, they didnt want him left behind if he didnt have to be.

I went saw the movie with him and had a great time. You never know who is out there, even the employees aren't always trustworthy.