Kumon, the tutoring franchise, is expanding in Manhattan; two branches have recently opened (at 80 Carmine St., nr. Varick St., 212-647-8670; and 381 Second Ave., at 22nd St., 212-683-1232). The flat $150-per-subject monthly fee is a bargain compared with private tutoring. Here’s how it works: A student comes to Kumon two days a week for instruction and does topic-intensive worksheets on the other days. The company says 50 percent of math students and 33 percent of readers study above their grade level. One mom whose 5- and 7-year-old children have been Kumon-ing for two years says both are well beyond their public-school peers; more important, they’ve developed good study habits and enjoy their schoolwork.