By Denny Dyroff, Entertainment Editor, The Times Longwood Gardens Spending a day enjoying an annual Christmas event in the Brandywine Valley has become a family tradition for many. Visits to sites such as Brandywine Museum, Longwood Gardens and Hagley Museum are highlights of the family holiday calendar. Longwood Gardens (Route 1, Kennett Square, 610-388-1000, www.longwoodgardens.org) has shifted...