It was either a book of short stories or one of those where each chapter is an episode from the character's life told as a standalone incident.
First story or chapter was told from a child's POV. The child's mother was either dying or had recently died, and the father had joined some kind of New Age-y support group. They drove to a meeting of the support group which was held in a large house. While the meeting was going on, the children of the group were left to their own devices in the garden. There was some kind of dangerous play structure - a tall tree, tree house, climbing frame, rope swing etc. There was an older child who was bullying the smaller ones. One of the children fell off the dangerous whatever-it-was. The child narrator did not find out whether or not the other child had been killed - they left the house straight after the accident.
Hoping this rings a bell with someone! Wondering whether it might possibly have been an Alice Monro short story.