Christine Anne Mayor, of Wales, UK, hid a small recording device inside her 15-year-old son's cuddly toy and captured the carers making 'unacceptable' remarks to the vulnerable teenager.
The tape reveals voices shouting and swearing at Kyle Thomas, calling him at 'dirty boy'.
On the tape, voices can be heard shouting 'dirty boy' at Kyle Thomas, as well as a series of expletives.
Following a lengthy investigation and formal hearing by Rhondda Cynon Taf council's Children's Services, four employees have been given formal warnings but have remained in their posts.
Single mother Christine said her severely autistic son had been visiting the Nantgwyn centre for four years, spending on spent around four nights a month at the Aberdare home to allow his mother brief respite.
But Ms Mayor said she decided to take matters into her own hands when she heard her child using abusive language.
She said: 'Kyle cannot talk properly and he suffers with Echolalia, which means he repeats a lot of things people say.
'I knew he was getting these words from somewhere, so I decided to take matters into my own hands in October last year.
'When I brought it home and listened to the recording, I nearly had a heart attack.'
'What I heard was completely inappropriate language and I was not happy with the tone of their voices. I felt that the way they spoke to him was really horrible.'
A lengthy transcript of the recording was made and the matter was initially handled by South Wales Police before being handed over to social services.
She said: 'As my son cannot talk properly, he couldn't tell me what was wrong - I felt like I needed to be his voice.
'He would curl up into the brace position whenever we tried to drop him off at the home, and he'd always back away from the door. It could have been happening for years.
'I put my trust in these people and they've let me down.'
Ms Mayor is now looking to complain to the ombudsman about the four staff members being reinstated.
A council spokesman said: 'Rhondda Cynon Taf council can confirm that it received a complaint from an individual in relation to a residential children's facility within the county borough.
'Following receipt of this complaint, a full investigation was conducted and appropriate action concluded in line with council policy.'