You can use the EventManager on the grid to disable/enable the event. So you would disable the event, set the Width on the column and then re-enable the event. It's typically a good idea to do this in a try...finally block so that the re-enabling is always hit.

        private void ultraGrid1_AfterColPosChanged(object sender, AfterColPosChangedEventArgs e)
        {
            UltraGrid grid = (UltraGrid)sender;
            grid.EventManager.SetEnabled(GridEventIds.AfterColPosChanged, false);

            try
            {
                // Copy the width of the columns to the child band
            }
            finally
            {
                grid.EventManager.SetEnabled(GridEventIds.AfterColPosChanged, true);
            }
        }