Master Detail with two DataGrids

I have a form with 2 DataGrids.
The first is bound with a parent table and the second is
bound with a child table. The update and delete rules of
the relation are Cascade.
When I change a parent's key column value and after I
click on the child DataGrid, the child DataGrid becomes
empty. Now, if I changed the parent's key column value and
click on another parent's row and after click on child
DataGrid, it's ok.
What do I need to do?