The screws on the back of the door control the door locking, not the child locks on a Lag 2. AFAIK, the child locks are totally electronic on the 2 and controlled with the switch below the electric windows on the driver's door handle.
I'm guessing all the child lock on a Lag2 does is enable the 'deadlock' on the rear doors, thus disengaging the internal handle, so it's difficult to tell if the central locking servo on that door is actually working at all. Normally the doors can be opened from the inside whether locked or not.
My rear N/S door will lock, but not unlock with the central locking - if I open it from the inside, it remains unlocked for the external or internal handle from there on. If this is not the case with yours, I'd look at the exterior handle linkage, as suggested. If it does, then you're in the same mystery boat as me