It would have been perfectly reasonable had KLM insisted that the girl be accompanied by her own doctor if she's that vulnerable. It's considerate of the airline to even provide supplementary oxygen at a charge when by the tone of it all, had the little girl died on board, the airline would have found themselves with a damages claim at worst and bad publicity at best even had the parents signed a waiver.
No doubt the £660 will be provided by a donation, which is probably the law of intended consequences. The Dolphin Academy on Curacao cooperates with the Dutch Make a Wish Foundation which permits children with a life threatening disease to live their dream of swimming with dolphins.