A 12-year-old Milwaukee boy is accused of sexually assaulting a four-year-old girl at a YMCA. The boy appeared yesterday before a Children's Court judge on charges including first-degree sexual assault of a child. He could be sent to a juvenile institution if found delinquent at a hearing later this month. Most of the encounter was caught on tape by security cameras at the YMCA. The delinquency petition says a manager saw the assault taking place on a security monitor, ran to a weight room and found the boy pulling up his shorts. The petition says the boy hit the girl, pushed her down, kicked her, pulled at her pants and choked her with her scarf.