The U.S. Customs Service this summer will begin a limited drug screening program of key personnel, including many of its 1,100 workers in the Washington area.

Commissioner William von Raab is studying recommendations of a task force he set up last fall to deal with potential drug problems among the Customs Service's 12,000 workers, many of whom work in drug enforcement or support programs.

Although details of the testing program have not been approved, a customs spokesman said yesterday that the agency anticipates testing most new hires starting later this year. They will be given urinalysis tests to determine if they use marijuana, amphetamines, cocaine, heroin, morphine or PCP.

Later, key employes in law enforcement and inspection, as well as pilots, boat handlers and top staffers will begin taking the tests. An official said yesterday he expected von Raab would be "first in line" to take the tests when they are begun.

Last Thursday, employes got a memo alerting them to the testing program. In it, von Raab said he believes the Customs Service is largely drug-free, but the tests must be made because "our primary enforcement mission is drug interdiction . . . and we cannot tolerate the illegal use of drugs."

Many other agencies involved in law enforcement, national security or safety work are watching the customs program and are expected to adopt something like it for some of their employes. People:

Senior Executive Association's executive director Blair Childs is leaving the association next month, after five years. He's one of the best amateur skiers in the area and plans to remain here. He directed SEA's efforts to win a big back pay settlement for executives and worked with Congress and the administration to win major improvements in government travel and per diem payments.

The extensive fan club of Gloria Pratt, one of the highest-ranking career employes at the Labor Department until her retirement in December, will hold a special luncheon for her April 2 at the National Press Club. During her more than 27 years at the department, she helped set up or run many of its major domestic and foreign programs.

The newly formed House Legislative Assistants Association has elected a tripartisan board (Democratic, Republican and Boll Weevil staffers) to plan its business and social sessions. Bill O'Neil is president of the 400-member HLAA. Other officers include Joe Kolter, Ted Chang, Sylvia Binkowski, Leigh Anne Preston and Peter Davidson.

The U.S. Postal Service has named Frank S. Johnson Jr., to be assistant postmaster general for public affairs and communications. Johnson had been with the National Aeronautics and Space Administration. Retiree Raises:

Sen. Paul Sarbanes (D-Md.) has introduced a bill that would guarantee a full cost-of-living adjustment next January for more than 2 million federal retirees and survivors. Retirees were due a 3.1 percent COLA at the beginning of this year, but the increase was canceled because of the deficit-reduction act. Checks In The Mail:

The National Association of Letter Carriers health plan, appropriately enough, is the first health plan to actually put refund checks for overpayment of 1985 premiums in the mail. The NALC health plan has 214,000 members. Refunds range from $24.84 to $61.52.