parents. the child is incapable of making a legally binding contract (especially this one). the parents are the legal decision makers. anyone with a decent conscience (and without specific religious opposition like jews who can't be buried without all their organs) supports organ donation.
it's simply not an issue. the child is brain-dead for all intents and purposes. sucks, but maybe they could have saved 10 other children with her blood and organs and tissue. do you know how many babies need skin grafts? i don't, but i imagine there's plenty.