A 35-year-old man has been arrested on suspicion of murder following the deaths of a brother and sister in Bodelwyddan, Denbighshire.
Caitlin and Aidan Samuel were pronounced dead at hospital
The children, named locally as Caitlin Faith Samuel, five, and Aidan Thomas Samuel, three, were found just before 2300 GMT on Sunday.
The two children were taken to Ysbyty Glan Clwyd where they were later pronounced dead.
North Wales Police said post-mortem examinations would take place later.
The children were found at a property in Coronation Close after a man inside the house called 999.
North Wales Police confirmed that a man was arrested at the house but did not reveal whether he was related to the dead children.
Forensic officers are examining the children's house
Speaking at a press conference in nearby St Asaph, Detective Superintendent John Chapman, who is leading the investigation, said: "Shortly before 11pm last night, police were called to an address in Bodelwyddan.
"Once at the property, officers searched the house and discovered two children.
"The ambulance service were alerted and the two children, who were brother and sister, were taken by ambulance to Ysbyty Glan Clwyd in Bodelwyddan. They were pronounced dead at the hospital an hour later.
"A 35-year-old man was arrested at the property on suspicion of murder and currently remains in custody awaiting questioning."
Floral tributes and cards have been left in Coronation Close
"Police inquiries into the incident are well under way and I, along with my team of officers, are currently in the process of trying to establish the circumstances surrounding the deaths."
Both children were pupils at Ysgol y Faenol school in Bodelwyddan.
A mother whose children attend the school said: "The kids were told when they arrived at school this morning.
"They don't really understand what has happened, and there has been a lot of tears. Everybody is devastated."
In a statement, head teacher Martin Edwards said: "Our whole school community has been deeply shocked and saddened at this tragic loss.
"The two children were bright, popular and happy pupils at Ysgol y Faenol and will be very sadly missed by their classmates and the staff at the school."
One resident, who did not wish to be named, said the children were well known in the area.
He said: "They used to come knocking on my door asking for sweets, they were very nice children.
"It's just such a shame. I came home from work last night to see all the police on the doorstep. "Nobody knew what had gone on until the news this morning. It's just awful, everybody is still just trying to take it in."
Reverend Berw Hughes, of St Margaret's Church in Bodelwyddan, said the village would be "hit hard" by the deaths.
"Of course, something like this would be difficult wherever it happened, but it will hit this community very hard because it is the sort of place where people still know and talk to each other."
North Wales Police have appealed for anyone with information to contact them on 0845 607 1001 (Welsh language line) or 0845 607 1002 (English language line).