Courtney Stewart was questioned by police following complaints about the stunt and says she was even attacked on her doorstep after sharing the image.
It shows the two-year-old – whom she says is small for his age - inside the machine with an adult’s hand holding the door.
The 21-year-old told the Daily Record: “He loves the washing machine. We took a picture and it was a laugh.”
Stewart had been looking after the little boy at her home in Erskine, Renfrewshire, when she was reported to the police by her horrified friends.
One source told the newspaper: “It was terrible to look at. For anyone, let alone someone who is also a parent, to take a picture like that and stick it on Facebook for a laugh is unbelievable. It’s no wonder people are angry about it.”
A defiant Stewart has refused to apologise for the incident – despite claiming she was physically attacked because of it.
She says the machine was unplugged and insisted: “He is clearly laughing in the pictures.”
Courtney Stewart posted this pic of her Down's Syndrome son inside a washing machine... and says 'It was a laugh' pic.twitter.com/kmKtIjiR3L— John Laverty (@JohnCharlesLave) July 22, 2015
Courtney Stewart, 21, refused to apologise for her actions and claimed the two-year-old boy climbed in of his own accord— Badru WISE de BîðøÑ™ (@BadruWISE) July 22, 2015
'He loves the machine. We took a picture... it was a laugh.' Courtney Stewart, 21, has been called names and assaulted over the photograph.— Renkian (@_Skendong) July 22, 2015
A spokesman for Police Scotland confirmed to Huffington Post UK officers had “attended the home and relevant checks were made, but no criminality was established. “
He also confirmed a 42-year-old woman was arrested over the alleged assault on Stewart and appeared at Paisley Sheriff Court on Tuesday.
A spokesman for Renfrewshire Council said: "We can't comment on individuals but we are vigilant on any issue which could potentially affect the welfare of a child."