Daniel Reed Cunningham, 33, also was charged Thursday with distributing a controlled substance on the Aug. 25 flight from Amsterdam to Detroit.
He is expected to appear in court next week for his arraignment.
The girl's mother, Beate Turner, told FBI special agent Terry Booth that Cunningham seemed upset when her daughter became restless and began squirming and crying on the flight. Cunningham offered to give the girl apple juice three times before Turner accepted, according to the agent's affidavit. The girl suffered no serious injury.
Turner later noticed the juice was bitter and foamy and had blue and white specks floating in it. Ten days after the flight, she took the juice to University Laboratories in Novi, which confirmed the presence of Xanax, a prescription medication used to treat panic attacks and anxiety, the FBI said.
The drug's side effects include lightheadedness, fatigue and drowsiness. The Food and Drug Administration hasn't approved it for children under age 18, the agency said.
Cunningham has denied drugging the child in statements to the FBI. His attorney, Neil Fink, declined comment.
Cunningham also has been charged with importing more than 100 tablets of a non-narcotic controlled substance into the United States on a different flight in October. The tablets included Xanax and Valium.
Northwest spokeswoman Mary Stanik said Cunningham was hired in 1998 and worked for the airline until last Dec. 30. She said she could not discuss his case.