My DD went to one of these for 21/2 - 4 year olds. Sometimes there were as many as 12 girls. But the teacher always had an assistant, and the Mums had to wait in a back room (out of sight of the LOs). That way if anyone needed the loo, got upset etc, the helper would just give the relevent mum a shout. Are you sure she is completely on her own - I think that the recommended ratios are about 1:5 at that age.
I think you are fussing rather. At my dd's class (she's three) we all wait outside, and if any of them need a wee they are sent outside and we take them. I, of course, always make sure my daughter has a wee first, because I am practically perfect dd loves her class, I think it's good for the children to be independent, and much nicer for the teacher not to be watched beadily by eight fiercely partisan mothers!
My understanding is (and feel free to shoot me down in flames) is that classes like this have to have a certain number of 'open' sessions a year where parents can watch. Some kind of child protetion issue or something, tho i am really not an expert in this. Other than this I would be happt for my DD to attend a class, tho i would make sure she went for a wee first.
We were never allowed to stay and tbh I think this was as well. Stopped us all fussing and worrying about whether our dd's goood-toes-naughty-toes was getting as much appreciation as the little show-off from over the road . I just took dd for a wee before it started. And then I reckoned she was in the hands of capable people who had dealt with 4yos before.
My daughter's ballet classes have always had a teacher plus an assistant who did the escorting to the loo etc. The mums (or, on Saturday mornings, mostly dads) have a peaceful half an hour nattering and drinking coffee but at the end of term, we all get to watch the class. The girls find it harder to concentrate when their parents are in the room, so I can understand why the teacher is encouraging parents to leave.