The woman was seriously wounded in the attack near the local sports ground in Vise, and died early today of her injuries, the Belga news agency said.
The boy, whose age was given as nine or 10, was the woman's nephew and godson.
The Liege public prosecutors office was investigating the killings but had not established how many people were involved in the attack, it said.
The victims were shot several times with a pistol, it added.
Other reports citing Neven said it appeared the killer or killers waited for the victims but there was no obvious motive for the attack.