A 10-year-old girl admitted strangling the 13-month-old daughter of her father's girlfriend while baby-sitting last August. The girl, whose name was not released, will be sentenced Dec. 5 in Los Angeles Juvenile Court and could be held in the California Youth Authority until she is 25 years old, Deputy Dist. Atty. Mitch Harris said. He said he could not discuss the details of the case because the girl is a juvenile. "It's tragic," he said. "A 10-year-old child committing a murder is unheard of." The murder occurred when the girl's father and his girlfriend went out Aug. 6, leaving the child in charge of the woman's baby. The girl admitted to a petition that alleged the murder had taken place because guilty pleas are not taken in Juvenile Court, prosecutors said.