Mark the First record in child grid whenever you change the selection of Parent grid.

In child data source - >> methods

public void linkActive()
{
    FinderRecords   finderrec;
    super();
     finderrec   =   FinderRecords_ds.getFirst();

    if(finderrec)
    {
    FinderRecords_ds.findRecord(finderrec);
        FinderRecords_ds.mark(true);
    }
}