We are seeking an authentic castle for lodging with fun activities for teenage children!
Try this directory for castles throughout England, Scotland, Wales and Ireland: http://www.celticcastles.com/. I have also found useful travel advice and reviews through http://www.theguardian.com/travel.
Haven't stayed there yet (early October) but we are staying at Hazlewood Castle and resort outside of York. We did the Great Yorkshire Tour which was a great rate for two nights mid week. Not sure about activities for kids.