AT THE BROOKLYN Children's Museum the other day, 15 sober grown-ups took an oath to do their duty by yet another federal agency dispensing money to support American culture. The newly created agency is the Institute of Museum Services.HEW Secretary Joseph A. Califano Jr., who used to roam the 73 year-old Children's Museum as a boy, administered the oath. Museum services are in his department's bailiwick. Under the law, the services are rendered not only to museums of art, history and science but also to zoos, botanical gardens, and aquariums. Lee Kimche, the museum official in charge of the new institute, estimates that there are about 9,000 of them -- all in tough financial situations.

The reason museums need more money than they are able to collect from private sources is that most of them have ceased being places where people come to look at objects. They have become active educational institutions as well, which try to present art, history and science with flair and showmanship, to reach out to their communities and make special efforts to train teachers and attract children. The National Endowments for the Arts and for the Humanities, as well as the National Science Foundation and other federal agencies, can make grants for special research, programs or projects. They cannot help with operational funds -- money for the additional guards and professional staff, utility costs, postage stamps and so forth -- that increased activities and inflation demand.

Household expenses at the Brooklyn Children's Museum increased 90 per cent in the past seven years, Lloyd Hezekiah, the director, has said. Craig Gilborn, director of the Adirondack Museum at Blue Mountain Lake, N.Y., provides an illustration: He got a $50,000 grant from the National Endowment for the Humanities to mount a special exhibition of the history of trapping and hunting in his area (6 million acres of protected wildneress -- an area larger than the state of Massachusetts). But the exhibition will probably be poorly protected. The recent increase in the minimum wage will force Gilborn to reduce the seasonal 30 guards who control summer crowds --most of them retired teachers -- to a mere 20.

The Museum Services Institute is supposed to help out with this sort of thing, but only $4 million was appropriated for this year. That makes the question of how it should allocate these obviously insufficient funds all the more acute. The needs are as compelling among the prestigious art and natural-history museums along the country's seashores as among the small town historic societies in the midlands. The National Endowments have often been criticized for favoring the established "elite." Now President Carter and Joan Mondale, who seem to press for a more even distribution, are accused of "flabby populism" and of seeking to spread federal art funds "throughout the land like jam."

We think that is nonsense. Simplistic distinctions between "elitism" and "populism" are just that: simplistic. The same may be said for geographical differentiations: Excellent and "true values" are not confined to the East and West coasts of the country. The National Museum Services Board needs to take the trouble to seek out the worthiest and most promising institutions -- and to distribute its meager funds in a way that will encourage deserving enterprises that are outside the mainstream of cultural fashion.