Police found the bodies of five-year-old Zaraiyah-Lily Headland and three-year- old Andreas Headland on a bed at a Yanchep home on October 20 last year.

Their father Jason Craig Headland, 36, was also found lying on the floor with self-inflicted wounds.

The WA Supreme Court heard on Thursday that the cause of death could not be ascertained but the children had been drugged with a sleeping tablet and asphyxiated.

The court also heard Headland's relationship with his partner had deteriorated and he wanted to take the children away from her.

"I'm going to break your heart into 50 million little pieces," Headland told her over the phone, adding it would be the last time she would speak to the children.

She also heard him giggle before he hung up and she went to police for help.

In a note police found at the house, Headland addressed his estranged wife saying she would have to "live with this for the rest of your life".

Headland's wife Anatoria said in her victim impact statement that her children were bubbly and excited to learn, describing them as her heart and soul.

"Two peas in a pod they were and polar opposites," she said.

"They completed each other and they completed me."

Ms Headland described her daughter as a "determined little girl" and her son as a "brave soldier".

Since their murders, she has suffered anxiety and depression, and is scared that if Headland is released he will come after her.

"You make me sick. I'll never forgive you," she said, addressing Headland in her statement.

Headland's defence counsel said Headland had a strong sense of family values and was an active member of the indigenous community, including being a mentor to young Aboriginal men and coaching a women's football team.

Justice Lindy Jenkins recognised Headland's early guilty plea but questioned the extent of his remorse, adding he had not given an account of how the children died.

His lawyer said Headland had no memory of what happened.

Prosecutor Amanda Forrester said Headland showed no remorse or contrition, describing him as self-centred and lacking insight or empathy.

He may have been a loving parent, but he "destroyed every remnant of that" by his actions, she said.

"There is no such thing as a comparable case," Ms Forrester said.

Ms Forrester urged Justice Jenkins to hand down a long non-parole period to demonstrate society's "utter condemnation" of a "grotesque act of family violence".

Justice Jenkins said Headland actions were vindictive and his note was "callous, shallow and self-centred".

"You had a duty to love and protect them. Instead of protecting them, you used them as weapons," she said.

"You made your children pay with their lives in order to inflict pain."

AAP