This is a full-on family annihilation, not only the children were murdered but their mother, too, and the perpetrator committed suicide on the spot.
He was seen filling a can of petrol and it seems he lurked waited outside his PiL's house where his wife had taken herself and the children for safety, jumped into the car passenger seat as she was taking the children on the school run and sloshed the petrol over his family. Police haven't released details of how the fire started but zippo lighters are cheap and readily available.
Hannah managed to get out, alight, yelling that 'he' had thrown petrol on her. A passerby tried to get the children out and received burns in the unsuccessful attempt. He was out of the car and stabbing himself to death.
Those, people, are not the actions of a loving father.