Actually, it has been shown that children are by far the most receptive
audience for new music.  They don't know yet from dreadful, and are usually
fascinated by the odd noises.  They don't understand harmony yet, just
sound and expression, and haven't yet formed opinions/prejudices yet about
what they're hearing.

I don't know Birtwhistle's music, I must confess, but I see that Gawain is
about the story of Gawain and the Green Knight.  A friend of mine wrote a
successful illustrated children's book on this story.  I'm sure he doesn't
know there's an opera written about it!  I'll have to tell him.

Dave Runnion
Mallorca