Shots In The Dark by Hazel Townson
Danny and Georgia have both lost a father (one a detective, one a criminal) in the same shooting incident. Each bereavement leads to further disaster and the pair meet by chance in a hospital waiting room. They then turn their joint attention to the drug-dealer responsible for the crime and his son Jamie, who is a pupil at Danny's school. In the devastation of their loss, Danny and Georgia cannot then avoid thoughts of possible revenge on the downtrodden and unsuspecting Jamie.