The Midnight Gang by David Walliams. Ages 8+.
The Midnight Gang (2016) is set in a children's hospital ward. Tom has been admitted with a cricket ball injury. He discovers that at midnight, when the night nurse is asleep, the children on the ward sneak out for secret adventures organised by the kind hospital porter, Porter. Each child in the ward has a wish — something they dream of doing that their illness or situation makes impossible. The Midnight Gang makes each wish come true.
The villains are the horrible Matron and her sister Miss Swine (familiar from Grandpa's Great Escape), who run the ward with cruelty and contempt. The novel is a celebration of imagination, courage and the idea that a child's dreams matter — even when they are seriously ill.