Her partner and Arthur's father, 29-year-old Thomas Hughes, was found guilty of manslaughter, after his son suffered an "unsurvivable brain injury" at Tustin's Cranmore Road home in Solihull on June 16 2020.
Tustin was also found guilty of poisoning Arthur by force-feeding him salt-laced meals, with Hughes cleared of a similar charge.
It has since emerged that Tustin - described as "wicked" by Arthur's grandmother - had "salt thrown at her" by other inmates while on remand at HMP Peterborough for her trial, a criminal justice system source said.
An HMP Peterborough spokesman said: "We do not comment on individual prisoners."
Arthur, described as a previously happy, smiley boy, was also "deprived of adequate food and water", forced to stand in isolation up to 14 hours a day, and repeatedly beaten, punched and smacked.
As well as being targeted with salt, Tustin was also threatened with violence by other prisoners in the run-up to her trial.
At a pre-trial hearing in April 2021, Tustin's barrister said her client had been receiving "significant and substantial threats", although it is unclear if she was at the same prison at the time.
Speaking at a hearing on April 15, Mary Prior QC said: "She has had a significant deterioration in her mental health.
“There are significant difficulties for her in that she is currently housed in an environment where she is not receiving medication.
"And she is receiving substantial and real threats - and violence."
Ms Prior said Tustin's legal team had "done all we can" to urge the prison authorities to "make it plain" to safeguard their client's health and wellbeing, including writing to the governor.
She added: "There are weekly conferences and they consist almost entirely of an overwhelmed young lady who is receiving significant and substantial threats and minimal medication."
Ms Prior told the presiding judge at the time, Recorder of Birmingham Judge Melbourne Inman QC, she had felt it "necessary to say it in open court" because "nothing is changing".
Speaking at the time, Judge Inman said: "The prison estate, I know, are very good at maintaining medication and treatment for those in their care.
"All I would say is, if there are concerns and problems, the court should be kept informed and, if necessary, we can make inquiries to assist.
"That is not meant as a criticism of anybody but, obviously, if matters of concern are raised with the court, we will do everything we can to assist - to make sure they are resolved."