Fifteen-month-old Charlie Timbrell died at Alder Hey Children's Hospital in Liverpool yesterday evening, after a flat fire at a property in Maes Y Groes, Prestatyn.
The blaze had already claimed the lives of Charlie's mother, Lee-Anna Shiers, along with her nephew, four-year-old Bailey Allen and two-year-old Skye Allen.
North Wales Police have been given an extra 36 hours to question two people who were arrested on suspicion of murder.
A special memorial service has been held in Prestatyn to celebrate the lives of five people who were killed in a house fire last month.
Funerals will be held for the five people who died as a result of a flat fire in Prestatyn last month.
Two people arrested following a fire in Prestatyn in which four died have been released from police custody.