'Don't hurt him': William Tyrrell's mother begs kidnappers to bring him home

William Tyrrell's biological mother has made an emotional plea for the person who took her son to bring him back home.

Karlie Tyrrell broke down in tears as she spoke publicly about William's disappearance for the first time in an exclusive interview with Seven's Sunday Night.

"Don't hurt him," she said.

"Just let him come home. Please."

William, who was living with a foster family along with his older sister, was four years old when he vanished from the front yard of his foster grandmother's home in Kendall, NSW, in 2014.

His biological mother Karlie had two more boys after William and said she is desperate to have her family back together.

"He hasn't even met his little brother yet," the mum-of-four said. "It's not fair. This isn't fair.

"I want my kids to be together. They're meant to be together, like brother and sister."

The identity of William's foster carers has never been made public, but the family - along with mum Karlie and William's biological father Brendan Collins - have all been ruled out as suspects by police.

Speaking anonymously to Sunday Night, William's foster mother said she was having a cup of tea on the balcony when she realised she could no longer hear William playing in the garden.

Karlie said she believes the foster family "failed" to look after her eldest son.

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"I don't want to blame the carers, but they were responsible for looking after him and they failed," she said.

"She went inside and made a cup of tea. If that's the case, okay, that's an accident, and that's unfortunate.

"But it doesn't make any sense to me. Kids don't just go missing."

The distressed mother is adamant her son is still alive.

"Where else could he be? He's not in the bush," she said. "I feel like whoever has him needs a bullet."

William would be turning seven in June.

Police have offered a $1 million reward for information that leads to William being found.

Anyone with information is encouraged to call Crime Stoppers on 1800 333 000 or use the Crime Stoppers online reporting page.