I have finally found a Play Centre which will take my ds's allergies seriously. I stay with him, there are no latex gloves, balloons, peanuts etc.
Today somebody brought 2 balloons in when picking up their child for the clean up. During clean up all the children sing songs with a staff member while we all do a job. So ds is unattended by me for a whole 10 minutes, which was the 10 minutes the balloons appeared.
I zoomed across the room to pick up ds, and called to another mum to remove the balloons. The girl who was playing with them then had a total melt down, it went on for over 20 minutes, it was still going on as I left. I got evil looks from her mother - presumably she had brought in the balloons.
So it seems everyone understands about bob's allergies right up until the point that it makes their own life a tiny bit difficult.