In this context, it would seem to mean a police officer who has special responsibility for locating children who are truanting from school, and ensuring that they attend school.
In the US, a school may employ a truant officer who is concurrently a constable. Their main role is to ensure that the children of the district are safe and attending school. The truant officer will go to places that children are likely to hide when they are truanting. Then they will detain (not arrest) the child and return them to school or home as appropriate.
So I understand that this child witnessed his father murder his mother. He was absent from school for three days, until he was found by a constable who had special responsibility for ensuring that children are safe and at school.