William Tyrrell’s biological mum Karlie has a devastating message for her missing son.

One minute, William Tyrrell was playing in his foster grandmother’s garden. The next, he had vanished.

It’s been three and a half years since the three year old disappeared and now his biological mother, Karlie Tyrrell, has spoken to the media for the first time.

In an interview with Melissa Doyle on Sunday Night, Karlie broke down as she relayed a message to her missing son.

“I love you so much. I miss you,” she said.

She also delivered an emotional plea to her son’s kidnappers.

“Don’t hurt him. Just let him come home. Please. It’s not fair. This isn’t fair,” she said, sobbing.

“Just let him come home. He hasn’t even met his little brother yet.”

Karlie was eight months pregnant with her fourth child, William’s younger brother, when police came knocking at her door asking if she knew where William was.

She denied accusations she was involved in his disappearance.

"No. I was at Blacktown shopping. I showed the police my ATM receipt, my docket, everything," she said, in tears.

Asked whether she thought William's foster family should feel guilty about what happened, she said yes.

"They were responsible for looking after him, and they failed. She went inside and made a cup of tea. If that's the case, like, OK, that's an accident, and that's unfortunate, but it doesn't make any sense to me. Kids don't just go missing," she said.

Karlie had four children with ex-partner Brendan Collins, who she met when she was 19 years old - an older daughter, William and two boys who live with her now. The pair are no longer together.

"It wasn't working and my kids come first, so I had to leave," she told Sunday Night.

William's older sister was taken into foster care first, because Karlie was in a domestic violence situation and tested positive to a drug screen.

Listen to The Quicky debrief on the truth about William Tyrrell's parents, and what happened after the three-year-old's disappearance. Post continues below.

"[It was because of] bad choices and I wasn't ready I guess, to have my daughter. I wanted to do everything I could do to be the best mum I could be. I'm still trying to do everything I can but I don't know what else to do," she told the show.

William joined his older sister with her foster family, a professional couple living in Sydney's north shore, when he was nine months old.

They were both playing at their foster Grandmother's house in Kendall when William, then three and a half years old, went missing.

Until last year, the fact that William was under foster care was a secret. It was made public in August last year after New South Wales Supreme Court of Appeal ruled it a matter “of legitimate public interest”.

If William is still alive, which Karlie passionately believes he is, he'll be seven in June. He shares his birthday with his Karlie.

"I just pretend it's not happening. Yeah. Like, I want to celebrate it, in a way. Like... But then I... It feels... like, doesn't feel right," she says.

"Especially, like, for my four-year-old - "So, let's go celebrate, like, your missing brother's birthday!" I can't do that to them."