A 2-year-old foster child was found dead of apparent beating injuries in the Brentwood Village neighborhood yesterday evening after being left in the care of a 12-year-old boy, District police said.

Mykeeda A. Hampton was pronounced dead at 5:52 p.m. at Children's Hospital, 10 minutes after ambulance attendants arrived at her foster mother's apartment at 2500 14th St. NE. They had been summoned there by an adult relative who went to the apartment on the second floor of the three-story red brick building and found the unconscious child, police said.

According to sources, the baby sitter told police he became angered and struck the child on her chest after she urinated on him. The baby sitter was not identified.

An examination at the hospital revealed bruises and scabs that appeared to be from injuries that occurred before last night, police sources said. However, neighbors said they believed that the child had arrived at the house only recently. Police said an autopsy was planned this morning and that the cause of death remained under investigation.

The child's foster mother, identified by sources as Geraldine Stevenson, was interviewed by police as to where she was when the death occurred.

According to sources, the mother originally told police that she had left the home about 9 a.m. to have her car repaired.

Police said about four other children were in the apartment when ambulance attendants arrived, but authorities said they were unsure whether any of them were foster children.

Neighbors in the three-unit apartment building said Stevenson had four foster children, all young girls. They said she has four natural sons, three of whom live in the apartment.

Neighbors said they were stunned by Mykeeda's death, describing Stevenson as a caring and attentive adult. They said she has cared for foster children for about a year and that she had quit a government job at the Interior Department to have more time with the children.

Priscilla Adams, 37, who has lived in the third-floor unit of the building at Brentwood and 14th Street NE for six years, said she believed that Mykeeda had arrived at the house about a week ago. She said she had rarely seen Stevenson outside playing with Mykeeda.

"I'm just shocked. She is a good parent and often took the boys and girls to church," Adams said. "Nothing has ever happened here like this. Everyone in this apartment building is like family. We all get along and watch out for each other."

Adams said that when she went home yesterday from her contracting job in McLean, Stevenson was outside the apartment talking to police. "She told me that one of her foster daughters had gotten sick, and she thought she was dead."

Adams' husband Alvin said Stevenson's sons "were nice kids, they were very easy to get along with."

Police said details of the case were unclear last night. They were uncertain how many times Mykeeda had been struck or how long she had been injured when ambulance attendants arrived.

Charles Seigel, a spokesman for the city Department of Human Services, said foster parents are allowed to leave their homes but, he said, he believes that they are required to submit plans to the department indicating who will supervise foster children in their absence. He said he did not know whether Stevenson had submitted such a plan.

The city's supervision of foster parents came under scrutiny last year after a March 1986 house fire killed five children, including four foster children placed there by the city, while the foster mother was away.