Reading a statement to the High Court at Auckland, Malcolm's mother, Savanna Bell, said she was haunted at night by visions of her son's "little body" being smashed.
"You're a liar, a coward, a monster," she told Welsh.
"You killed my baby," she said, adding that in her opinion that was murder.
According to the summary of facts, on June 23 last year Welsh threw Malcolm with significant force towards a couch while more than a metre away.
Malcolm hit the back of the couch, the wooden frame of which was thinly covered, and fell to the floor.
"He was immediately rendered unconscious and did not move," Justice Christian Whata said.
At 7.32pm, Welsh had called Bell, saying Malcolm was having a seizure and needed an ambulance.
She recalled in court how she had run faster than she had ever done before, desperately trying to get home.
"The world around me was spinning out of control. I just had to get to my son."
As she ran, she dialled 111 and spoke to an ambulance call-taker.
When the ambulance arrived at 7.43pm, paramedics would describe the unresponsive boy's breathing then as "slow and laboured".
Bell said Welsh had failed to tell the truth about what happened and she sometimes wondered if her boy would have received the help he needed if Welsh had told the truth.
She later watched Malcolm slip away in hospital, the court heard.
After the sentencing, Detective Senior Sergeant Geoff Baber said Malcolm's life was "cruelly taken from him".
"He was a vulnerable little boy and he should have been safe in his home."
The Herald has previously reported that Oranga Tamariki had been contacted with concerns about the welfare of Malcolm before he suffered fatal injuries.
After his death the agency initially declined to comment before an Auckland manager then said they had been working to support to him and his whānau before his death.