THE D.C. DEPARTMENT of Employment Services is making the frustrating task of getting a summer job even harder for poor youngsters. Under a new city requirement, teen-agers must bring along a parent or legal guardian when registering for federally funded summmer jobs. Last year, teen-agers had only to bring proof that their family's income was below a certain level and have a parent sign the application form. An employment services spokesmansays some teen-agers forged their parent's signature or brought false documentation of family income. So the new law requires the youngsters to bring their parents with them.

The trouble is that many poor children have trouble convincing a parent to stand in line with them for the standard two to five hours of waiting. Only two weeks ago were evening registration sessions -- Mondays and Thursdays from 5 to 8 -- arranged to accommodate working parents. But many parents still are reluctant or unable to go. The results are evident in the numbers.

As of May 9, 15,643 youths had registered, against more than 17,000 by April of last year. Nearly half of those registered are not expected to meet the qualifications for the program. The employment department estimates that it would take a registration of about 30,000 youngsters to find a sufficient number of youngsters eligible for the Comprehensive Employment and Training Act jobs.

The change in the rules, by the way, is because of a change in federal CETA regulations, according to the city. But Jim Bryfonski, manpower development specialist in the federal Labor Department, says no such change has been made. He notes that federal regulations require only a parent's signature attesting that the information given is true.

The District government's troubles in registering needy city youth for what summer jobs do exist is all the more disturbing because fewer private employers than ever appear to be hiring summer help this year. The Greater Washington Metropolitan Board of Trade reports that it has been able to get pledges for less than half of the 2,900 summer jobs it had sought. This is the first year in the 16 years of the board's program that the number of summer jobs appears to be falling. This means there is an urgent need for the city to get as many poor youth into government-sponsored jobs as possible. To that end, the parent-with-child rule at job registration centers should be abandoned.