As ever, it is an education reading MN.
I am surprised at the large numbers of posters of the 'How awful is nakedness/nudity! Cover up your children at all times! There are paedo perverts everywhere!' stripe. But since such people exist, particularly it seems in Britain, those of us who see nothing particularly wrong with nakedness need to take account of their opinions and feelings.
That does not excuse the attendant who interacted so badly with OP and her child; no excuse for that. I would have been very annoyed. But still, yes, we need to compromise so as to avoid offending each others' sensibilities.
There are some caveats. I am an old man. I often have taken grandchildren, boys and girls, swimming, both in UK and elsewhere. Sometimes I have had force majeur to take granddaughters into men's changing rooms. Make them shower with their cossies on? Nope, at least not if they don't want to. The child is the most important person, imo; that decides matters there. If some perverted man were to get his rocks off somehow by looking at my granddaughter, I would not be too pleased (and if I found out, he had better watch out!), but still, on balance, and given my own views, I will not force such discomfort on a child, minor though such discomfort may seem to some.
(I feel the same, mutatis mutandis, about my own comfort. In communal-open-shower changing rooms, I do know there are some people who dislike the sight of naked bodies such as mine, aged and wrinkly as it is. But, well, tant pis; I do not enjoy showering wearing a cossie. So, tough on them.)
As for compromise, there does seem to be an answer in extra 'mixed' areas. One pool I visit (not in UK) has three changing rooms: 'Men', 'Women', 'Mixed-or-no-gender'. There I use the latter, despite disagreeing with the word 'gender' as used so. There I can go with my partner, children, grandchildren, whoever, and not worry about people getting upset at anyone's nakedness. (And my comments above about pervy men apply there too.) There is an added advantage, too, in that it seems very few people class themselves as 'mixed-or-no-gender' so there is always lots of space. True, I have had some funny looks (guess from whom?), but, again, tant pis for that.
So, compromise with added mixed changing -- at least until everyone loses their hangups about nakedness. I am not holding my breath on either of these outcomes in UK, so, meanwhile, those who dislike/fear nakedness, think just a bit more about those of us who do not, children especially included ...
... Do not you tell us or our children to cover up, and we will not tell you or your children to strip off. Deal?