to be fair, they were evacuated during the war in the Lion the Witch and the Wardrobe. And then in Prince Caspian they were on their way back to school. In the Voyage of the Dawn Treader 2 of them had to stay with Eustace's family because they were too young (the others were all travelling, and i do agree that was unfair.)
The Silver Chair: they were at school
The Magician's Nephew: Diggory lived with his mum next door to Polly. Uncle Andrew was awful, but not a parent. And they were playing together at home so no parental involvement needed.
Horse and His Boy: set during Wardrobe
The Last Battle: some were adults. All ended up... sorry, no spoilers.