Yes, I think it is very extreme to attach a nanny cam to the clothing of a child. I mean how would you explain that to a child? Or the nanny if she found out? Moreover, what would the school or daycare center think? I can completely understand hiding a nanny cam in your home, perhaps in a teddy bear or something. But not on your actual child.
I've used nanny cams in the home before and I firmly believe in letting the nanny know she is being watched - otherwise it's just an invasion of privacy. You know? I've found this pretty helpful website about nannies and nanny cams
which helped me make the decision - but it's still a really controversial subject.