Authorities say Huckaby killed Sandra Cantu, a playmate of Huckaby's daughter. The girl's body was later found stuffed in a suitcase in a nearby irrigation pond.
The plea comes after increased pressure by prosecutors. Her lawyer, Sam Behar, has repeatedly requested more time to review evidence in the case.
Huckaby could face the death penalty if convicted.
The judge scheduled her preliminary hearing to start Sept. 24.