The foster mum of a baby girl left severely malnourished and suffering rickets after being fed a vegan-only diet by her biological parents has told of the developmental problems and trauma the child has suffered.

Police alleged the father, 34, and mother, 32, - who cannot be named for legal reasons - fed their 20-month-old girl a strict diet that lacked enough nutrients for her to thrive.

They are now facing jail time after pleading guilty to failing to provide for a child causing serious injury.

On Thursday, a victim impact statement from the girl's foster mum was read out in the Downing Centre Court during a sentencing hearing.

The statement spoke of the child's medical and developmental challenges caused by the malnourishment.

The child was laying in a hospital bed, being fed through a tube, when the foster mum started caring for the child in August 2018.

"I remember thinking, 'how terrifying'," she said.

"She couldn't sit up .... she couldn't say any words ... she couldn't hold her bottle."

Chevron Right Icon 'She couldn't sit up .... she couldn't say any words ... she couldn't hold her bottle.'

"This crime has had a long-term impact on her development," the foster mum said.

She said the child has made "great progress" but worries about the longer physical challenges she may face.

The court heard the child will soon be two-and-a-half and is only 76cm tall and wears size 0 clothes, which is standard for a 12-month-old.

"People are shocked at how small she is ... she immediately gets up and sits away from other children," she said in the statement.

She also said the child requires constant blood tests and often screams and cries.

"I worry about the longer physical consequences," she said.