Alice Mackey, 42, admitted killing Annabel Mackey in Kingsley, Hampshire, in September 2023.
The two-year-old went missing from her home on 10 September.
She was found a short time later unresponsive at Kingsley Pond.
Annabel was taken to hospital in a serious condition, but died the following afternoon.
Mackey entered the plea at Winchester Crown Court.
She entered a not guilty plea to a charge of murder, which the court heard was acceptable to the prosecution, as reports by two psychiatrists showed she was in a state of “diminished responsibility” at the time of the incident.
Judge Christopher Parker KC adjourned the case for sentencing on 6 October.
He remanded Mackey into custody until then.
