A father who wanted to punish his wife for leaving him used their young children as “weapons” to “break her heart into 50 million little pieces” by callously killing them.

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

Their father Jason Craig Headland, 36, was also found with self-inflicted stab wounds, including three to his neck.

In the WA Supreme Court on Thursday he was sentenced to life behind bars, with a minimum of 31 years to be served for his “grotesque act of family violence”.

The court heard that the children had been drugged with a sleeping tablets crushed in their juice and then asphyxiated.

Justice Lindy Jenkins described Headland’s actions as vindictive and the most extreme act of selfishness.

“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 and loss on your wife.”

The court heard Headland’s relationship with wife Anatoria had deteriorated and after she demanded a break 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. Ms Headland went to police for help.

In a note found at the house, Headland told his wife she would have to “live with this for the rest of your miserable life”.

Justice Jenkins described the note as “callous, shallow and self-centred”.

Ms Headland 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.

“What you did was heartless and selfish.”

The defence lawyer submitted that Headland had had a strong sense of family values and was an active member of the indigenous community, including being a mentor to young Aboriginal men.

Justice Jenkins questioned the extent of Headland’s remorse, adding he had not explained how the children died.

His lawyer said Headland had no memory of what happened.

Prosecutor Amanda Forrester argued Headland had 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” with his “grotesque act of family violence”, she said.

“There is no such thing as a comparable case.”

The state would have argued Headland should never be released if he had not pleaded guilty to their murder.