Mind you, I'm not sure any of the Marlow holiday books stand up to responsible parenting scrutiny, even by the standards of period children's books where adults have to be hapless, absent or actively evil in order for plots to happen.
In Falconer's Lure, Patrick has been badly injured enough to be off school for months after a fall from steep cliffs, whereupon he promptly takes Peter and Nicola up the same cliffs after young birds of prey, Peter gets stuck and nearly falls, and they have to get the coastguard to rescue them. The first their parents know of it is when it's in the local paper. Then Peter shoots and kills Patrick's hawk killing rabbits, and could probably have shot Nicola or Patrick too.
In Peter's Room, Patrick puts an old gun to his head and is about to pull the trigger without realising it's loaded before Nicola knocks his hand away, meaning it discharges out the window and nearly hits Rowan.
In The Marlows and the Traitor, not only are Peter and Nicola nearly washed out to sea in a dangerous surge on holiday, and Nicola spends all her free time with a much older male stranger she never mentions to her family. they then trespass in an empty house while their mother is away for a weekend, narrowly miss being killed by spies and ex-Nazis, are kidnapped, Ginty and Nicola nearly drown faking Peter's death, Lawrie is badly hurt and hospitalised after jumping off a bus she doesn't have a fare for into traffic, and Peter shoots and kills a man. And their mother knows nothing about any of this.
In The Thuggery Affair, Peter, Patrick and Lawrie get involved with a drug-smuggling, violent local gang, Lawrie narrowly escapes sexual assault, one gang member is stabbed and killed, Peter and Patrick are both threatened and duffed up, and Patrick joyrides with a gang member who is killed when their stolen car crashes, while Patrick is thrown clear.
In Run Away Home, Peter and the supposedly responsible Giles sail across the Channel in a barely seaworthy boat to return a fugitive child to French soil, Giles is knocked unconscious on the way home, Peter can't really handle the boat solo, so navigates for an entire night through marina traffic by luck and torchlight, whereupon the boat breaks up on the shore and they both nearly drown.
And I'm sure there's more I've forgotten. 