Witnesses say they heard at least eight gunshots around 8:30 p.m.
The victims, who were on the porch of a house in the 1200 block of East 55th Street, were both struck in the head.
The man, who is in his 20s, was hospitalized in critical condition. The toddler, Joshua Montes, was pronounced dead at County USC Medical Center, according to the coroner.
Police said the boy and his uncle were apparently unintended victims of random gunfire.
The suspects fled the scene on bicycles, according to CBS2′s Kirk Hawkins.