I can't imagine what they're going through, it's utterly tragic, but sadly I think the chances of finding a suitable donor on such a short timescale are very slim - it would need to come from another child of a similar size, and for heart transplants the donor has to die in quite a specific way (be declared brainstem dead) so that the heart is still beating. I just don't think there can be that many tiny babies dying like that every day, on the donor register or not.