In my experience Linq To SQL requires the Primary/Foreign key relationships established in the DB. Otherwise, you'll have to do a join.
Not sure if this applies to your situation, but it may...
Linq To SQL Without Explicit Foreign Key Relationships
UPDATE:
It appears that you can get what you're looking for without establishing the Primary/Foreign key relationships in the database. In the LinqToSQL designer set primary keys on both columns on each table. I also had to turn off Auto-Sync on Parent Table's column that I assigned as a primary key.
Auto-Sync is described here..
Instructs the common language runtime (CLR) to retrieve the value after an insert or update operation.
Link:
http://msdn.microsoft.com/en-us/library/bb386971.aspx