That is a valid question. I don't have children and have no desire to change that, so any answer I can give you is only theoretical, but based on what if:
If I had a child old enough and mature enough to make that decision, I would let the child decide, since it would concern their life and their future. In the case of a younger child I would look for every other possibility of treatment. There are many other options today. I know of a few cases of things as complex as transplants that were successfully done without blood. People are used to do transfusions because that's the way some treatments are normally done and let's be honest, blood is a great business - you give it for free and then it's sold for a good amount of money - but there are alternatives. Why not try them first and see if they work?
Should the alternatives not work and the only thing left to do is a blood transfusion, there are other questions to ask. Is the child going to be cured or is the transfusion going to maybe just postpone the inevitable? If it's just postponing death, that would be by how long, and what kind of life would my child have? It's not black and white.
Ultimately, if it came to that, the law in France is very strict on matters that affect minors. If there is a conflict between the parents and the doctors regarding the treatment of a minor, it goes to court, so in that last case scenario, the decision wouldn't even be mine.
Medical care is a very complex subject and while it's easy to say "I would do this or I would that", I would have to be in that situation to really know what I would do. That's the most honest answer I can give you.