The man is not related to the two dead children, North Wales Police said.
Detective Superintendent John Chapman said: "We would appeal for anyone who may have seen anything suspicious in the Maes y Groes area of Prestatyn yesterday evening to get in contact as soon as possible by calling us on 101.
"We are deploying family liaison officers to work closely with the next of kin at this difficult time.
"There is a team of dedicated officers working on specific lines of inquiry in relation to this incident."
Firefighters were called to the scene at 10pm on Friday night and recovered the five people. A joint investigation was launched by North Wales Fire and Rescue Service and North Wales Police.