SURELY IN MOST QUARTERS where little people dwell the word has spread that on a night soon, the streets will be swarming with under-age mendicants in weird attire. And however unimpressed some adults may be by the thought of it all, the advent of Halloween does bring on an annual centroversy of considerable intensity - to wit, which night are the kids supposed to go out? Sunday the 30th? Or Monday? It isn't only parents and their children who want to know, either; many others would like to find out so they can scheduled dinner parties, nights out or trips.

But because Hallowe'en is the special province of children, there is no national declaration on trick-or-treating dates, no formal proclamation from the Great Pumpkin. On top of that, in this multijurisdictional region the local governments have been reluctant to declare themselves for one date or another when weekends have been involved. This year, as best we can determine, the traditional date - Oct. 31, Monday night, the eve of All Saints' Day - is prevailing. If it'll help in settling any neighborhood decisions, we'll endorse Monday, too - though , as one astute editor on our city desk suggests, the matter really cries out for a regional approach , a uniform code if you will - that could be hammered out by the Metropolitan Washington Council of Governments. But as of late last night, COG members were out bobbing for apples and were unavailable for comments.

At any rate, if children must go through with this rite, there are some ways in which adults can help make the night a success. They can make sure that costumes are not made of highly flammable materials; that face masks allow children to see, breathe and speak easily; that bright colors or reflecting tapes are used; that adults escorts are on hand; and that all treats are brought home for inspection.

We would be remiss, of course, if we did not add a word here, about another custom that began 27 Hallowe'ens ago, when a Sunday School class in Pennsylvania decided to collect in behalf of others as well as themselves. As a result, more than 100 million children throughout the world have been helped by UNICEF. The funds collected aid needy children without regard to creed, color, nationality or political belief. UNICEF is one of the few organizations that have been consistently able to help children on all sides of political or military conflicts.

Moreover, the need for this humanitarian effort increases each year. The number of sick, undernourished and uneducated children keeps growing, and the diaster areas of the world constantly cry out for medical aid, food, emergency supplies and compassion. UNICEF, by the way,is not financed through the regular U.N. budget, but through voluntary contributions. There will be thousands of local children seeking your help this hear for a successful campaign. They deserve it. For information on the activities in this community, you can call 547-0204, where volunteers will be on duty today from 10 a.m. to 2 p.m. and again on Monday.