The cop allegedly tasered the 94-year-old earlier this year.
The child was seen lying face-down and gripping on to the roof while she drove around.
She's accused of planning to kill him with plots involving an axe, a pillow and a hitman.
Hundreds have been evacuated from the area around northern Perth.
Erin Patterson's screenshots of an old text message conversation have begun circulating.
It comes after Australian homicide detectives arrested and charged her with murder earlier this month.
The two toddlers were aged two and three.
The 11-year-old boy was hit by an SUV.
CCTV footage shows the car splitting in two, and the men abandoning their two teenage dead passengers.