Sean Murphy, 3, was found unconscious by his mother Kerry in the front loader washing machine in their Hilton home just after 2pm on Monday after she woke from a nap.
She also discovered a dead cat in the washing machine with her son.
Detectives say the boy's death from fatal injuries is, at this stage, being treated as suspicious.
Ms Murphy and her defacto partner Craig Sheppard, both 24, were interviewed at length by detectives at Fremantle Police Station yesterday.
Their lawyer Terry Dobson said Mr Sheppard and Ms Murphy denied any involvement in Sean's death.
"The police made it quite clear that both of them are suspects,'' he said.
He said his clients were finding it difficult to cope with the loss.
Mr Dobson said Mr Sheppard was at work on Monday when Sean was found but was still considered a suspect.
"He was questioned and cooperated fully, he took part in a video recorded interview and after the family identified Sean, he took part in further interviews and gave a written statement," Mr Dobson said.
"The police made their views very clear to Mr Sheppard but he still cooperated with them ... despite some things being said to him that were, in his view, very very upsetting and quite awful, that the police considered that he could be responsible."
Mr Dobson said it was after Mr Sheppard realised he was under suspicion that he sought legal advice.
"There's no issue about the level of cooperation ... (but) you've got a death of a little one, and it's like husbands and wifes; the police automatically look at a spouse, so it's not to say there's anything in it, it's just a pretty basic policing tactic."
Mr Dobson said the couple were shattered by the boy's death, with family friends asking media camped outside their Harwood Street home to give the family space to grieve.
"On top of such a tragic loss and the grief that's come with it, they are also shocked and confused because of the circumstances of Sean's passing," he said.
Forensic police took samples and the washing machine from the scene. A post-mortem examination was conducted yesterday but is continuing.
"They are doing further testing today, which isn't unusual," Mr Dobson said.