You should really take a look at the [Depend] attributte. See the docs here
Basically, you apply it to the child list in your parent like:
public class Order
private IList<OrderLine> orderLines = new List<OrderLine>();
This will make OA delete the "dependent" objects when the an instance of the Order class is deleted. It works as a "cascading delete" in sql.
Hope this suits your needs..
Another way to do it if this is a 1:m relationship is to use DeleleOrphans... See the docs here