Nassau is moving to end a $2 million contract with a nonprofit that for more than 20 years has operated a program aimed at keeping children out of foster care, following a dispute with the county's largest public employee union over who should provide the service. Officials of the Family and Children's Association, of Mineola, say social workers help up to 200 families a year obtain mental health and parent coaching services to prevent abuse, neglect and removal of children to foster homes.