Child record picker

I have a contract table and a business unit table. The business unit table has level 1 BU and level 2 BU. A contract can be allocated to 1 or multiple BUs. The BU table also holds user BU information (1/0) to allow only contracts allocated to the user's BU to be visible. 

How do I now allocate BUs to a contract? When I establish a relationship the Level 1 BU and Level 2 BU are dropdowns disregarding the relationship between Level 1 BU and Level 2 BU. Also this expands the BU table with contract fields which is not my intention. 

Alternative solution? I feel like this is a quick fix, what am I missing?

------------------------------
Philip de Beukelaar
------------------------------