I don't know about BKK, but the Marriott Phuket Beach Club/JW Marriott Resort offered both family and adult Muay Thai classes as well as a Muay Thai tour. The instructor was excellent and the class was a good introduction to Muay Thai and a good workout. I found the current schedule of classes here: http://www.phuket.com/marriott/jw-maze.doc.

You might want to check to see if the resorts/hotels in BKK offer a similar program.