Pallavi Dhawan attended Wednesday night's gathering outside her family's home in Frisco, 25 miles north of Dallas. Dhawan was charged with murder after police officers making a welfare check at the house Jan. 29 found the body of her son, Arnav, in an empty bathtub. The body was wrapped in a cloth up to the neck, and there were empty plastic bags in the tub next to him.
Toxicology results are pending.
Dhawan remains free on $50,000 bail. Her attorney, David Finn, said she has denied killing the boy.
Arvan was a special-needs child, born with a brain cyst and microcephaly, a condition characterized by an abnormally small head, Finn has said.
The mother and her husband, Sumeet Dhawan, both shed tears during the vigil.
"When you lose a child, you lose a child. You really don't have time for those things," Pallavi Dhawan said, responding to the allegations against her. "Forget his ailments and everything, you know. Other than that he was a boy, a little kid."
One mourner sang "Amazing Grace." Some of Arnav's friends also attended. Cards and toys, including a stuffed bear, were left at the yard.