The 58-year-old woman appeared in Parramatta Local Court with her partner on allegations of assaulting and intimidating the child, who was in their care until being removed in November 2021.
In a clash covertly recorded by police in October that year, a loud bang could be heard followed by the youngster screaming in pain.
"She kicked me," the child was heard yelling.
"You're a bad mother."
The child, who is not William, later said they did not want to remain with the couple, leading to a rant from the mother about having a "get-out-of-jail-free card".
"You break our hearts, you lose the only parents you've ever known, the only parents who have fought tooth and nail for you," the foster mother was heard saying.
In a later phone call with another woman, the foster mother described the child as "feral" and said the youth "lost it" when they were kicked.
"I don't have words to describe how I feel on the back end of doing that," she said.
In a conversation with William's former foster father, the woman said she kicked the child hard during a "crisis point" for the family, pointing out the ongoing investigation into the young boy's unsolved 2014 disappearance.
"We've got this investigation hanging over our head," she said.
"We've got police looking at me … how do I live with that? Being looked at."
The woman said in another call on the same day that police had turned back to her because they had no other suspects in William's case.
"They're doing it because they f***ed it up," she said.
"They've cleared the guy who should still be a suspect."
After the call, she apologised to the youngster for the kick.
"What I did was completely out of line," she was heard saying.
"So I am sorry that I lost control and I'm sorry that I did that."
In May 2021, an argument was recorded in which the foster mother accused the child of throwing things around the home.
"If I see or hear you throw something around again that won't be the only thing that gets thrown," the woman said.
"You seem to only listen to me when I threaten to get violent with you."
A July argument over packing the dishwasher resulted in the foster mother threatening to pack up the child's things in a black plastic garbage bag.
"If I get that attitude ... I'm going to slap you across the face," the woman said.
"Do you want me to do that? Because you are pushing me big time."
The 58-year-old woman has pleaded guilty to assaulting the child on two occasions in January and October 2021 but is fighting two intimidation charges over a series of incidents between January and August that same year.
Her partner has pleaded not guilty to one count of assault and one count of intimidation over alleged conduct in November 2020 and October 2021.
Neither foster parent can be named for legal reasons.
The covert recordings from inside the foster parents' house on Sydney's north shore, their car and the pair's telephone conversations were made during a police investigation into William's disappearance.
The hearing continues.
William was three years old when he vanished from a Kendall property belonging to his foster grandmother on the NSW mid-north coast.
Police have sought answers in the years since his disappearance, including conducting a thorough search of the Kendall property in December 2020.
No one has been charged over his disappearance.
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