I've seen a few practice questions with this already...
A 42-yr-old man and his wife and 8-yr-old child are in a car accident. They are all BIBA to the hospital. The father is completely normal, conscious, aware, etc., but the mother and son are unconscious and need urgent transfusions for survival. The father is competent and absolutely refuses transfusions for both of them, and he even presents a card proving that they're all Jehovah's witness.
A general rule is that, in an emergency situation, you always transfuse JW children, even if the parents show proof of JW.
For adults, they always have their autonomy and can refuse Tx irrespective as to whether they're JW or not. If the father can show proof of the mother's JW, then the mother shouldn't be transfused, otherwise the physician can be sued for assault/battery.
In a non-emergent situation regarding transfusion of a child, don't transfuse, because, once again, irrespective of whether the child is JW, it is the parents that always are righted to give consent for any medical/surgical Tx for their children, and in this case, there's nothing immediately life-threatening. The physician can pursue a court-order, however, if there's enough time to do so and the pt's condition is serious enough where, although the situation isn't an emergency, his or her condition is serious enough to warrant it.