I am the Director of a Division and all leave requests are coming to me for approval. How can I re-route the requests to their direct supervisors?

If there is no lower structure of an organisational unit to map those supervisors as time approvers, one way to get around it is to assign those supervisors as secondary time managers.  Please note that both you and the supervisors will continue to receive all staff member requests in your Division, however, you reach an internal agreement where each supervisor selects and approves the requests of their direct-report.