OK, the reports are still unfolding, but...
The father of the children had this to say (and my emphasis, because reasons). This is a compilation from two sources, seemingly part of consecutive quote:
"I still love who Peter was," Mr Cockman told reporters in Margaret River on Sunday, marking the first time he had spoken publicly about the tragedy.
"If it wasn't for him, I wouldn't have Katrina, I wouldn't have her kids.
"It's not some random guy off the street who's taken them away from me - he gave them to me and now he's taken them away.
"If it had to happen, there is no better person than that.
"Peter didn't snap. He's thought this through. I think he's been thinking this through for a long time. All the kids died peacefully in their beds.
In the emotional address, Mr Cockman said he believed his former father-in-law had been "thinking this through for a long time''.
"All these kids died peacefully in their beds. The [police officer] that went through, he said they looked all peaceful. How the hell Peter did that I still can't figure out, but if someone did it ... he did a good job, he did a really good job.
''If someone was going to do it, I trust he did it right and he did it right."
"The kids went to sleep and now they are nothing."
These are not the words of a "grieving father" as the knee-jerk media portrays. Not even someone in complete denial. There is something more to this, idolising the FIL, basically congratulating him on 'a job well done'. It reads as if it was his own wish (to annihilate his family). There are other quotes there, effectively trying to cast some blame on the MIL (then ironically absolving her), and of course the media harping on about "embroiled in a bitter custody dispute with [the ex-wife, victim]™"
I have been following DV femicides/filicides for a very long time. In all those years, I have never heard statements like these from survivors - the ONLY time I have heard statements similar to this, is from the perpetrator (not saying he is the perp, but only ever a surviving perp says stuff like this). It does cross my mind as to whether there was an 'agreement' or 'arrangement' in place between this dude (children's father) and his former FIL. Which is speculation on my part, but experienced speculation nonetheless.