Responsible persons shall pay child support for a child placed in foster care through an agreement where legal custody remains with the parent or guardian pursuant to subdivision A 4 of § 16.1-278.2 or § 63.2-900, from the date that the child was placed in foster care. The agreement between the parents and the local board shall include provisions for the payment of child support. In fixing the amount of support, the court, the Division of Child Support Enforcement, and the local board shall consider the extent to which the payment of support by the responsible person may affect the ability of such responsible person to implement a foster care plan. If the responsible person fails or refuses to pay such sum on a timely basis, the local board may petition the juvenile court to order such payment.