I read it probably 15-20 years ago. I was very into Ruth Rendell at the time, but I don't think she was the author.
It's mystery/crime novel and the plot revolves around a play being put on in a big church/cathedral. All the actors are children. During the dress rehearsal a group come in and take the actors & adults hostage. I think they're looking for a specific child, a boy, and the adults hide him in a girl's costume among a group of hand maidens.
In it there's a line something like 'you don't want a real dress on stage, any more than you want a real murder' - possibly said as a wardrobe last pulls one of the hand maidens costumes apart.
