Creating relation in child tables

Hi

I have two child tables to the master table tblClients as per image below at the end; tblClientUniforms.ClientID<-Client.ID->tblClientRates.ClientID.

Now I want to restrict tblClientUniforms.Role values for a client to only those values that are in tblClientRates.Role for the same client. What sort of relation do I need between tblClientUniforms.Role and tblClientRates.Role and how do I create it? When I try to drag one role field on the other in SQL Server Diagram I get an 'The columns in table 'tblClientUniforms' do not match an existing primary key or UNIQUE constraint' error.

Thanks

Regards