To the Editor: Some years back while at a major advertising agency, I was part of a group brainstorming ideas on how a major advertiser could extend its reach among preteens. One Greenwich-type thought he had the idea of the century: sponsor little league sailing since, he pointed out, every preteen in his area was into sailing. One other member of the group sarcastically said, "Great idea, every kid in New York City is into it, too." I had a flashback of that experience when I read Daniel Wolkenfeld's letter suggesting that every family give $10 a month for each of their children enrolled in a school to that school to protect it from potential budget cuts and enhance the student learning environment. Great idea for the Upper East Side, but what about the rest of the city where so many families are below the poverty line and may need the money for food, school supplies and other essentials? Mr. Wolkenfeld is totally off-base if he thinks that every school would be capable of collecting a sizable amount of money that way. Ed Falk East 78th Street Letters have been edited for clarity, style and brevity.