If your children have an urge to start collecting stamps, tell them they could start with one of their own. The first United States stamp ever designed by a student was issued this month - National Stamp Collecting Month - as the US Postal Service's way of honoring family unity.
Designed in 1982 by then high school senior Molly LaRue of Shaker Heights, Ohio, the red, white, and blue stamp is a deceptively simple set of childlike stick figures with heart-shaped bodies and the words ''The United States of America'' scrawled down the side.
Miss LaRue's design was picked from a ''half-million or so'' the Postal Service received in response to its 1982 National Card and Letter Writing Week stamp-designing contest. The entries, winnowed down by a committee of educators and philatelists, turned up everything from dozens of rainbows to U.S.A. spelled out in sign language, computers, turkeys, ducks, and a snake.
That last item came with a note: ''I want to see a snake stamp because I like snakes,'' said the designer. ''Someday I would like to own one but my mother doesn't think the way I do.''
The committee sifted the enormous pile down to 85 entries, from which the postmaster general picked two - Miss LaRue's and a Santa Claus design for Christmas by nine-year-old Danny LaBoccetta, due out Oct. 30 (first-day issue, Jamaica, N.Y. 11431).
Commenting on why the United States, unlike a number of other countries including Britain and Canada, doesn't normally produce student-designed stamps, a spokesman said that decisions on what goes onto stamps are weighted heavily by the tastes of the collecting audience, ''most of whom prefer stamps to be art miniatures,'' he says.
''Generally, we stay with items like 100th or 150th anniversaries of organizations or individuals,'' he says, pointing to the Oct. 11 Eleanor Roosevelt birth centennial 20-cent commemorative.
''We get all kinds of suggestions on what to honor,'' the spokesman reports.
He also points to the stamp-producing process as a ''tricky'' one that will not easily accommodate many good designs. Miss LaRue's, for example, uses an eight-color combination gravure-intaglio press, with the red and blue applied by gravure and the black by intaglio.
The artist, however, was looking for simplicity - not complex printing problems.
Now a sophomore majoring in art therapy at Ohio Wesleyan University in Delaware, Ohio, Miss LaRue recalls that her art teacher ''advised keeping the design simple.'' That prompted her stick figures: ''I thought, what could be more simple than to draw as a young child would?''
Collectors of first-day-of-issue cancellations may send addressed envelopes to Family Unity Stamp, Postmaster, Cleveland, Ohio 44101, by Oct. 31. The cost is 20 cents for each stamp affixed on a cover. Personal checks are accepted.