Default field value inherited from parent

I have a parent table called "Requests" and a child table called "Request Items."  In the Request Item table, there is a field, "Billable?"  For any particular Request, the related Request Items would all be either "Yes" or "No" in the Billable field.  If this were always the case, I know that I could just put the "Billable?" field in the Request table and do a lookup on the child Request Item records. 

However, there are sometimes exceptions where some of the items are Yes and others are No on the same request.  

Question: I would like to put the "Billable?" field on the parent and also "Billable?" on the Request Items table.  The value of the "Billable?" field in the parent would become the default value of the "Billable?" field in the child Request Items record. So that user could change that value for each child Request Items record if needed.

Is this possible?