Ida Lee Evans, 31, and David Lee Delauder, 38, remain in jail on $100,000 bond each. Both are charged with death of a child by a parent.
Police said the 23-month-old girl died March 12.
State Police said Evans administered the methadone the night before, and Delauder made no attempt to call 911 or have the child checked out when he learned of the ingestion.
Investigators said the couple bought the drug on the street.