Who is involved in the accident and how old are they? Why is the accident seem particularly difficult for the two surviving children?

Jack, the oldest child and driver of the tractor, is twelve. David, his younger brother who is killed under the tractor, is seven. Phoebe, who at five is the youngest child, watches the accident as it happens. Jack is haunted by the fact he was driving the tractor and might have been able to stop it and save his brother. Phoebe is traumatized by witnessing the accident and seeing his brother pulled under the cultipacker.