I would guarantee that the parents hadn't read the calling letter.
A relatively new OC likes AOs and when I read one, I thought give it to a parent and they won't read it.
I do one pagers with relevant info like dates, drop off/pick up, cost, equipment/clothing needs and a broad outline of what's going on, but there's no major detail.
The complaints I've had is for repeats of the same activity. We do several canoe days over the summer and it's the same form bar a date change. Also the sheer volume of them for different things.
But what is a significant risk?
As for the incident in London, as I understand it wouldn't have made one iota of difference as the parents hadn't made it known what condition he had, the staff were well away from the cadets and as ever radios are rubbish for comms. Don't know if mobiles would work,