IF WE WERE IN THE business of handing out good-citizenship awards, one would certainly go to David Sutton, a seven-year-old apartment-dweller in Silver Spring who has been largely responsible for convicting a robber.

As recounted by staff writer Felicity Barringer last week, David was sweeping off his family's balcony one day last August when he saw two men "walking very suspiciously" nearby. He ran in, got his father's binoculars and watched from a window as the two accosted an elderly neighbor and took $137.

Three months later David identified one of the robbers from a police mug shot. Recently he showed up in county court to testify, but the defendant decided to plead guilty to a robbery charge instead of standing trial with David as the state's star witness.

Now, some parents among you may be less impressed by David's alertness and good memory than by the fact that he was sweeping the balcony in the first place. We are impressed by both. Even more commendable is David's clear understanding of how the system of justice is supposed to work. While waiting in court, he asked the prosecutor if he would "have to sit in the truth chair - that chair there next to the judge where you have to tell the truth?" We can't think of a better description of the witness chair, or for that matter a better example of how a conscientious witness can help fight crime. We wonder where he learned so much; no doubt from his parents, and perhaps a bit from television too. What matters is that he's learned.

"I'm proud of myself," David said. He should be.