Both De'Marquise Elkins, 17, and Dominique Lang, 15, are charged as adults. Authorities say Elkins fatally shot 13-month-old Antonio Santiago in his stroller as Elkins and Lang tried to rob the child's mother March 21 on the street a few blocks from her Brunswick apartment.
They entered their pleas in writing and waived arraignment before a judge that was scheduled for Thursday morning.
Superior Court Judge Stephen Kelley has said he wants a trial before the end of the year. Prosecutors weren't allowed to consider seeking the death penalty because Georgia law prohibits it for defendants charged with crimes before age 18.
The baby's mother, Sherry West, has said she was pushing her son in his stroller a few blocks from her apartment in Brunswick along the Georgia coast when two youths approached asking for money. When she refused, she said, one of them drew a gun and shot her in the leg before shooting her baby in the face.
An indictment charging Elkins on nine counts, including malice murder, identified him as the person authorities accuse of pulling the trigger. Lang was charged with seven counts, including felony murder, as an accomplice. Both face life in prison if convicted.
Police arrested both teenagers within a day of the shooting. Glynn County Police Chief Matt Doering has said information from Elkins' mother and older sister helped lead investigators to a small saltwater pond where they recovered a .22-caliber handgun suspected of being used in the slaying.
Elkins has also been charged in a second shooting and robbery attempt 10 days before the baby was killed. An indictment says Elkins pointed a gun at Wilfredo Calix-Flores while demanding his cellphone and wallet, then shot the man in the arm with the same caliber gun used to kill the baby.