I've been to the UK, and I can say this with certanity:

we speak AMERICAN

THEY speak English!!

I think the idea of getting them to work together or compete is polarizing the audience, a good thing.

As far as the ending have both groups yell simultaneously, look at the broken meter and say wow that was really loud.

I get the boys to shout, then the girls as a contest. Then I say "What if the boys and girls all shout together? Maybe we can get right up to 9!" A kid will always shout "No 10!!" I come back with, "Don't go to 10, 'cos it breaks the machine.... Don't do it....." Use reverse psychology on them.... This GUARANTEES they'll scream and shout til they go to 10 and the meter explodes! At which point I say "Ok.... Who went up to 10?" Most kids will put their hands up. I find this way works well, gets a great reaction, cos the kids 'get one over' on the magician.

Gary