Set filter to works better than a relation when it comes
to display only some itens of the child table

eg.

SET FILTER TO ALLTRIM(UPPER(FATHER_TABLE.type)) = ALLTRIM(UPPER(child_TABLE.type))

I put the code into the roll_event of the grid, so it happens
every time I move to a record on the grid of the father table

I am wrong ? I am forcing the system ?