Just write sql with parameter
select Material from Child.Materialname
where Child.Master = @master
Use this sql to reload second combobox on change event of first combo. Since you did not write witch combobox you are using I can't tell how this event is named I would guess ValueChanged
protected void combo1_ValueChanged(object source , ValueChangedArgs args)
{
FetchForCombo2WithMasterParam(combo1.Value);
}
And in FetchForCombo2WithMasterParam you would execute sql above with param from combo1.Value.