BY THE TIME I was in fifth grade. I almost had the official United Nations tour memorized. As an integral part of just about every year's schooling until then, product as I was of Manhattan's most progressive elementary schools. I was trundled off to the U. N. for a lesson in world harmony. By fifth grade, I had more to say about internationalism than Eleanor Roosevelt.

Eventually, I would join along on the official tour and know the end of almost every sentence before the interchangeably well-groomed Swedish, Ghanan, or Berundi tour guide completed the thought. The land on which the U.N. now stands was given by the Rockefellers, I would say to myself. The copper peace bell in the garden is made of the melted down pennies given by children all over the world, I'd mumble half-aloud. I would always know just where in the gift shop to find the colorful flags-of-all-nations combination paperweight and pencil holders.

What I didn't know in my fifth grade naivete was just what went on in those finely upholstered meeting rooms that we were whisked through only when they were empty. I greatly anticipated the day when I would turn fourteen, which qualifies you to actually sit in on a debate, and play with the little dial on the chair that chooses a simultaneous translation of the proceedings in any of five official U. N. languages. Until I turned 14, I was pretty much a regular on the tour and in the gift shop.

The view from the gift shop was upbeat as could be. In addition to the aforementioned paperweight/pencil holder, they had big fat pens with the U. N. insignia on it, and little dolls from various nations, that all smile at each other. What I didn't realize was that it was 1972, and the U. N. had spent almost a decade waffling on Vietnam. Soviet Jewry, and most of the other major calamities then facing the world. But it didn't matter: I had the ideals of international cooperation drilled into me well.

This week, the U. N. installed a photographic display of massacred Palestinians in the visitors' lobby, near where you stand to meet up with your official U. N. tour. It is an awful, one-sided display, which was cleared in advance with the Palestinian Liberation Organization's representative to the U. N.--and U. N. officials admitted that they would have delayed the exhibit had he not found it up to snuff.

Called "Brutal Massacre in Beirut, Lebanon, Sept. 16-18," it features 15 photographs of the mangled dead, including a young child in sneakers. The Israelis are not mentioned in the display, but there seems little question that further discrediting the Jewish state is the guiding purpose behind the whole exhibit.

The display is obviously just another example of the U. N. calling Israel to task for the sort of things it doesn't bother to--or cannot muster a majority vote for--calling other nations to task for, Ironically, the display goes up in the same week that the U.S. embassy estimated the death toll this past year alone in El Salvador to be 5,639, more than half of which were civilians. There seems, alas, no El Salvador photo display in the offing. Nor were there photo displays of the athletes slaughtered at the Munich Olympics in 1972, or the Jews killed in synagogue bombings across Europe this summer.

The current bias of the U. N. has shown itself time and again. It has not been a force for world good in many years now, and has greatly soiled its legitimacy by making anti-Zionist attacks one of the only agenda items on which it can approach consensus. But that is hardly the issue anymore.

The issue is simply that if the U. N. is going to hypocritically defy the good instincts upon which it was founded, it should at least keep things quietly confined to the plushly upholstered assembly rooms upstairs, the ones that you have to be over 14 to enter. For the sake of the noble ideals of international cooperation and world peace on which they were founded, let them at least spare the fifth graders and keep the political one-sidedness out of the visitors' lobby.