I wouldn't give a monkeys tbh. I think you need to get some real problems to deal with, instead of getting het up over (the horror) parent volunteers.
That said, if a parent volunteer actually started releasing confidential information about a pupil, I would be in to see the ht. (and usually the volunteer in question is then reminded of policy, thanked, and not invited back). Parent volunteers are all reminded frequently about their responsibilities towards confidentiality, and usually have to sign an agreement as well as CRB etc. They aren't usually allowed access to records as such, so it would just be day to day activity they would witness.
Mountain out of a molehill, much?
I'm not in favour of 'give a dog a bad name and hang him', so I would need actual proof of a helper being indiscreet with information gleaned from volunteering before I marched in and told the school which helpers could have access to my kids.
The tone of your post is gossipy in itself. If you have real concerns about confidentiality, then quit the juvenile 'mafia mummies' bollocks and make a formal complaint, not all this 'not my kids' pisstakery. A genuine concern needs appropriate action, otherwise you sound like you are perpetuating the ridiculousness (which, really, is bound to help in a playground stand-off.)
Why would you leave other people's children in that sort of position? Either you have a valid concern, or you are just as bad as the women you are bitching about. Not my kids, indeed.