The attack took place close to the local sports ground in Vise, the Belga news agency reported, citing local mayor Marc Neven.
"A man sitting in a car opened fire on a man, a woman and a boy as they were returning home," Neven said.
"The child was killed; the man, who was not his father, died later and the woman, who is his godmother, was very seriously injured," he said.
An investigation has begun as police search for the shooter.