March 28th, 2013

Blood transfusion from a child to a child

Hi!
I tried googling for children blood transfusion, I managed to find a few pages and even something to calculate the amount of blood that can be given, but none answers my question.
In my story, there are twins who have a rare mutation and need a very specific type of blood (O neg won't do it). Something happens to one of them, and the other one is the only one available to give him blood, since the parents are dead and no one else carries this specific mutation.
The twins are five.
The one injured has lost quite a lot of blood, he's in a state not yet critical but almost. What happens in that case? Would it be ok for the adoptive parents to decide that the other twin should give some blood to save his brother? Or is it strictly forbidden that a child would give blood, to the point that they would let the injured one die before taking the blood from a 5 year old?
What would be the maximum amount that could be taken from such a young child?
Thanks a lot!