The two-year-old was attacked by Alix Bluck, 22, who, along with her then partner James Bluck, 35, was supposedly caring for the child at the time.
One shocked visitor to the child's home in Kilmarnock, Ayrshire, described the boy as looking like a "Romanian orphan" as he lay on a mattress in an almost bare room.
The youngster eventually got medical help in March last year. He was transferred to the Southern General Hospital, Glasgow, where a CT scan revealed haemorrhages on both sides of his brain, which required surgery.
He now has permanent reduced vision in one eye.
A judge heard how Alix Bluck told police she had "lost her temper" with the child, while her 'ex' claimed he "never neglected" the youngster.
Alix Bluck admitted assaulting the child when she appeared at the High Court in Glasgow.
She and James Bluck also pleaded guilty to a charge under the Children And Young Persons Act.
Both will be sentenced next month.
The court was told the child often stayed with the Blucks at their home in Kilmarnock.
In March 2011, salesman William McLeod was at the house to measure for windows and doors.
Mr McLeod spotted a boy lying in a room, which had few toys in it and no furniture, apart from a mattress.
The salesman said: "I immediately got visions of posters or adverts years ago of Romanian orphans just lying or sitting on mattresses."
Mr McLeod also saw the toddler having a seizure and later reported what he had seen to his boss.
Another sales rep visited the Blucks the next day and she heard crying from upstairs, which James Bluck told her to ignore.
When the saleswoman said the boy could swallow his tongue, lorry driver Bluck replied: "I've checked before – he doesn't swallow his tongue."
The court heard she contacted social work, but, in the meantime, the boy had another seizure that led to him being taken to hospital.
Medics noted the child had multiple bruising as well as being "grossly underweight" and it was then he was sent to the Southern General.
The court was told Alix Bluck had pushed the boy twice at the beginning of February and that his head had hit the floor.
Prosecutor Sheena Fraser told the court: "She said this happened after she had lost her temper."