Karlie Tyrrell spoke exclusively to Channel 7's Sunday Night about the day her four-year-old son, who was in foster care along with his older sister, vanished from his foster grandmother's backyard in Kendall, NSW, in 2014.
An emotional Karlie, who believes her son was abducted, begged for his return.
'Don't hurt him,' she said. 'Just let him come home. Please.
The biological mother of William also has two younger sons, and said she is desperate to have her family back together.
'I want my kids to be together. They're meant to be together, like brother and sister,' she said.
'He hasn't even met his little brother yet.'
'Whoever has him needs a bullet.'
The identity of William's foster parents has never been made public, but his foster mum spoke anonymously to Sunday Night about the moment he went missing from the backyard.
'I'm yelling out 'William where are you, you need to talk to mummy, I can't see you, I can't hear you',' she said.
'He was nowhere. I was just standing there thinking, 'how could he just disappear?' He just disappeared, I don't get it.'
Meanwhile, a pregnant Karlie was eight months pregnant and out shopping for baby things, unaware her eldest son had gone missing.
When she arrived home, police knocked on the door asking if she knew where William was.
'They started searching the house, I was in shock,' she said.
While Karlie says she doesn't want to blame William's foster parents, she can't understand how he vanished.
'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.'
William's foster parents, Karlie and William's biological father Brendan Collins, have all been ruled out by police as suspects.
Police believe William was abducted by a pedophile and there is a $1 million reward for information that leads to William being found.
Anyone with information is urged to call Crime Stoppers on 1800 333 000.