I am thankful that I was never put in that situation, I don't know if I would have bowed to my parents pressure to refuse a blood transfusion. I suppose it would have depended on how old I was. What I have no doubt about though, is that if I had bowed to that pressure I would easily have been able to convince other people that it was 'my' informed decision.
I don't think any child bought up as a witness is capable of making an informed decision regarding blood transfusions since they are only informed of the witness party line on the subject as well as being subject to a great deal of pressure to conform.
If it were up to me I would take this decision away from every dub parent for the simple reason that the vast majority of children raised by witnesses will leave the religion. I can't think of anything more ridiculous than martyring children for a cult that they will in all probability leave once they are old enough to leave home.