Molly-Mae Wotherspoon, who was six months old, was attacked by an American pit bull in October 2014 at a house in Daventry, Northamptonshire.
Claire Riley, 23, had denied being in charge of a dangerously out of control dog resulting in death, but changed her plea on the first day of her trial.
Molly-Mae's grandmother, Susan Aucott, 55, admitted the same charge last week.
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Aucott, of Alfred Street, Northampton, was looking after her granddaughter at Riley's former home in Morning Star Road when the dog, called Bruiser, attacked her.
Molly-Mae suffered severe blood loss from head wounds. The dog was put down at the scene.
Riley, of Merrydale Square, Northampton, will be sentenced at Northampton Crown Court along with Aucott later.
The mother of a baby girl who was killed by a dog has admitted owning a banned breed.