surely this is only a big deal if we decide it is?
6 seems mighty young to sexualise - presuming that's the reason they are not allowed in the change rooms. It's not like it's a bar or massage parlour or something, it's just a room where people go to get changed.
I can understand that kids may become uncomfortable as they get older and their bodies change and so may appreciate some privacy. But in other cultures they don't really have these hang ups. Everyone just does stuff comunally and it aint no thang because that's just what you do. I'm not sure at what age it's considered appropriate for kids to be there, but certainly until they're old enough to do it on their own...?
in any event, pools and other facilities that are offering family activities probably need to think more about actually catering to their needs. if your pool doesn't, then maybe talk to management?