The mother of missing child William Tyrrell has broken her silence on her son’s disappearance, saying his foster parents “failed” to look after him.

Karlie Tyrrell has told Channel Seven’s Sunday Night her son’s foster family should feel guilty for William’s disappearance.

She also urged her son’s kidnapper to “let him come home” more than three years after he first vanished.

“I don’t want to blame the carers, but they were responsible for looking after him and they failed,” Ms Tyrrell said.

William, 4, went missing while he was playing in his foster grandmother’s front yard on the NSW Mid-North Coast on September 12, 2014.

His disappearance sparked a massive police investigation and nation-wide media coverage, but he still remains unfound.

“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,” Ms Tyrrell said.

Ms Tyrrell said after William went missing that she “felt like the worst mum in the world”.

“Because I haven’t come out and said anything, of course, people are gonna assume I don’t care,” she said.

“And that’s OK, because I know that I do.

“Their judgement doesn’t bother me, really. I’m not a bad person. I’m not a bad mum.”

Karlie Tyrrell: 'I didn't want to go on news being angry or anything, making anything worse, it didn't mean I didn't love him'. #SN7: https://t.co/KKdsYFVgcx pic.twitter.com/uOpkoa0thK

— 7NEWS Spotlight (@7NewsSpotlight) March 4, 2018

Ms Tyrrell, a mother-of-four, said she made some “bad choices” when William was taken to join his older sister to live in foster care.

“I just made bad choices and I wasn’t ready I guess to have my daughter,” an emotional Ms Tyrrell said.

“I had domestic violence and drugs and alcohol, marijuana.

“When I had to do a drug screen I tested positive after I had my children.”

Ms Tyrrell said she believes William is still alive and pleaded with her son’s kidnapper to not hurt him.

William Tyrrell was playing in the backyard of his grandmother’s home when he went missing. Photo: AAP

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

“Just let him come home. Please. It’s not fair. This isn’t fair.”

Ms Tyrrell added she is “still trying to do everything I can” to help bring William home, but admitted “I don’t know what else to do”.

“I didn’t want to go on news being angry or anything, making anything worse, it didn’t mean I didn’t love him.”

William’s foster mother, who still cares for his older sister, also told Sunday Night of her heartbreak.

“We need to know where he is, and we need to know what happened to him, ’cause we can’t live forever like this,” she said.

The NSW government last year offered a $1 million reward for any information that leads police to William, who turns seven in June.