I think that 10 is probably a sensible age for you to be able to make your own decision, say if you really didn't want to go ahead with it... then at least it'd be in your own hands. By that age I think the child has lived with the disease for long enough to know and decide what they want to do about it.
The chances are that the child will have spoken to their parents about it for a long time and between the two, they should be able to reach a decision on what they feel is best. At age 10, the child can still be easily influenced by their parents and the parents would, if it came down to it, be able to "talk them around" into understanding their point of view and why a transplant would be best for them.
Any younger than 10, then yes I do think that the parents should be able to decide what is best for their child, obviously after having talked to their them at length on the matter. Of course, the child even at such a young age will still be able to let their parents know what they want and have some influence on the decision that their parents will make.
However, the thought of surgery at such a young age must be very daunting for a child, and a child younger than 10 may be too put off by their own fear.
This is why I think that the parents of children under 10 should be allowed to decide, as the child isn't quite old enough to see past the fact that (s)he will be so scared.... but the operation could give him/her a new chance of life. Although this is something for the parents to keep in mind, I think it is right that they have to make the decision.