New York Acts to Coordinate Child Welfare
New York City Mayor Michael R Bloomberg announces restructuring of child welfare system to provide more comprehensive and cooperative monitoring of children living in troubled homes; move is in response to beating death of 7-year-old Nixzmary Brown, who was visited by caseworkers but obviously did not receive attention necessary to prevent fatal incident; Bloomberg reports that city will devote additional $16 million to Administration for Children's Services; changes represent attempt to bolster existing system rather than dismantle it; increases in staff are planned across all agencies dealing with child welfare and families; photo (M)