Jermane Johnson was just 18-months old when he was killed in his Kansas City, MO home, by the youngster who was annoyed by his tears.
The 16-year-old girl who was supposed to be looking after them had fallen asleep.
Police spokesman Darin Snapp said: "I've been in law enforcement for 20 years, and it's the youngest suspect I can remember. It's extremely rare."
"She said she got angry because he would not stop crying and she held him under the water until he stopped crying."