BOERUM HILL - A new proposal aims to keep Brooklyn and city schoolyards open after school, but officials say that could cost millions of dollars in maintenance and staff. New Yorkers for Parks say the Department of Education owns more than 900 acres of schoolyards that could serve as a place for children to play after school. One member of the custodians union, however, says keeping a schoolyard open after hours means more work and more pay. Those who support extending schoolyard hours say the extra space could mean higher grades and a better quality of life for Brooklyn children. City Council members say if schoolyards were opened to the public, the available space would be larger than Central Park. They say they will continue to work out the details of the legislation.