Could not find where to set it up.
In the Relationships Graph, double-click on the relationship and set the checkbox. I've attached a screen shot.
their is a flag in the relationship for delete related records when parent is deleted
This is a powerful tool and if misused, it can have major negative consequences for your data by deleting large numbers of records that you may not have wanted to deleted. Nothing in the relationship graph will tell you which relationships have this option specified either.
So use with caution and document the use of this option consistently and clearly. I've recommended to my coworkers that they add a text box to the relationship graph where they list every relationship that has this "delete" option specified.
Retrieving data ...