The young girl, who has a rare genetic disorder and could not walk or talk, or eat normally, survived the alleged attempt to kill her but needed months of hospital care.
Her 42-year-old mother, who cannot be identified for legal reasons, admitted injecting her daughter in early 2022 and knowing it could have killed her but she denies it was attempted murder.
"(The woman) had reached the end of her ability to cope with her daughter", prosecutor Ben Stanwix told the Supreme Court in Perth on Monday.
"She injected (her daughter) with insulin because she was deliberately trying to kill her."
Mr Stanwix said the child's condition, in which her immune system attacked her brain and central nervous system, left her with significant neurological damage and severe cerebral palsy.
Her mother tried to give her the around-the-clock care she needed but failed to attend specialist appointments, where treatment "was readily available".
"Despite the accused's efforts, you hear evidence in the trial (the girl's) care was suboptimal," Mr Stanwix told the jury during his opening submissions.
Instead, the mother sought help from a general practitioner who provided treatment to the child and supported the mother.
Towards the end of 2021, the woman told the doctor her daughter was deteriorating, "nearing the end of her life," Mr Stanwix said.
"That wasn't true.
"Rather than getting the ... level of care she needed, the accused decided to euthanise her."
Mr Stanwix said the woman had decided "the best thing for everyone was for her disabled child to die".
"Then she set about trying to murder her."
The woman did this by allegedly convincing the doctor to give her a prescription for insulin, which can fatally lower a person's blood sugar levels when improperly used.
Mr Stanwix said she administered the attempted lethal dose while her husband was at the beach with their other children.
Her alleged plan unravelled when her family returned home sooner than expected and found the child in distress.
The girl was taken to hospital after the "accused faffed around" and "deliberately held back information".
Medical tests revealed the girl had dangerously low blood sugar.
"From the moment (the woman) injected her daughter with insulin ... (the girl's) situation was a life-threatening medical emergency," Mr Stanwix said.
"This was simply a very sad situation where (the woman) had had enough."
The trial continues.
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