And you should look at individual circumstances rather than making a blanket judgement?
We went to a family party yesterday. There was a massive 12ft trampoline with safety netting on it, and the kids were having lots of fun bouncing on it together.
Normally I am a one child at a time on the trampoline person because my 5 year old likes to do seat drops, pike, straddle and star jumps which he learns at gymnastics and are best done solo.
However yesterday my 5 year old went on with a couple of 3 year olds and they were having a whale of a time. Lots of little bounces, pulling faces at each other and making lots of "boing boing" noises etc. I did think of my one at a time rule but I thought they are not jumping or moving fast, they are of a similar size, they are all behaving nicely, their parents didn't mind, we were ready to step in if things changed. They are having fun.
Also one of the three year olds has a speech delay and yesterday was the first time I heard him say something that I actually understood.
He was repeating what the other kids were saying. His dad, who was watching, looked a bit shocked at that as well. 
So anyway, one of the other mums come over because her three year old wanted to join in (I would have made my 5 year old come off) but her one at a time rule was not budging and her child wasn't allowed on, and then told us about the dangers and accidents of more than one at a time and trampolines in general.
I didn't agree or disagree with her, just acknowledged what she said, and the kids carried on. Fair enough, up to her. This thread isn't about us disagreeing.
But it got me thinking. Was I right to let them have fun with a small amount of risk or should I have played it safe?