I would imagine we're talking about situations where the child needs blood soon, but not necessarily immediately.
Yes. Surgeries, high risk births, etc.
My understanding is that, at least in most of the U.S., if a child needs an emergency transfusion due to being in an automible accident, they get it. Now. The squawking of their cult-member parents is just an annoying bit of background noise in the ER.
What now?! That's illegal, if the parents of a minor child are cognizant, present and expressed their decisions. In all the hospitals I've worked in/with, I've never heard of random ED staff overriding parental decisions in the manner you described. Shudder, I hope that's not really happening. Good lord. (I mean, good for the jw kids, bad for medical ethics.)
http://virtualmentor.ama-assn.org/2006/10/hlaw1-0610.html