You are not being overprotective at all.

Let me tell you that only 2 years ago (so I was 23 and I am a qualified primary school teacher) I babysat for someone doing a similar thing to your friends. They were having a Christmas party and wanted someone to entertain and mind all the kids in the front yard (secured area) and the children ranged from about 10months to 12yrs old. There were about 12 kids and even after I roped in the help of the two 12 yr olds, I still struggled and told her that I wouldnt be able to do that again.

I was also annoyed because I only got paid the same amount I would to look after one child and I was looking after 12!

I didnt feel that the kids were safe enough and the responsibility of looking after such a range of ages was too much.