The presiding judge of the District's family court concluded yesterday that police guidelines were not followed when a 12-year-old boy charged with kicking another child in a playground brawl was placed in a cell at D.C. Superior Court for at least five hours.

Judge Ricardo M. Urbina said he had concluded after nearly a weeklong investigation that the police officer who processed the child's arrest on April 1 made an "error in judgment" that resulted in the "unnecessary detention of this youngster."

Urbina said the police officer failed to ask for the necessary approval from a supervising officer to take the child to the court cell blocks. Such a request likely would have been denied because both the prosecutor and a social services worker had recommended that the child be released, Urbina said.

"My concern is that no youth who may be particularly vulnerable to the conditions brought about through detention be detained . . . ," Urbina said in an interview. "Exposure to a cell block situation is unhealthy."

Confining youths in holding cells at the courthouse has been the focus of intense scrutiny since it was revealed last year that an 11-year-old boy was sexually assaulted in a courthouse cell.

The 12-year-old boy detained April 1 is described in police records as 5 feet tall and weighing 80 pounds, although his mother said yesterday he weighs little more than 50 pounds and is several inches smaller than 5 feet. The boy had no previous arrests. Young offenders are held in the same cell as juveniles up to 18 years old and those charged with felonies.

"He was scared halfway out of his wits," said his mother. "He told me there were some guys in there for murder and other things."

According to his mother and police records, the boy was arrested April 1 at his Southeast home and charged with felony "assault with a dangerous weapon, shod foot" in connection with a March 22 playground fight with an 8-year-old.

The 12-year-old's mother said yesterday her son's kick was in response to a punch from the other boy.

Guidelines, issued last year after the assault on the 11-year-old, require police officers to get permission from the commanding officer of their district before detaining a youth who is older than 10 but younger than 13. The guidelines also urge consideration of releasing a child to a relative, having a police officer wait with the child in the courtroom or asking for the hearing to be expedited. One source said yesterday that the police officer in question was not aware of the guidelines.

A police spokesman said yesterday the department had not yet completed its own investigation of the incident and will not comment until that time.