One of the best things about a festival is the late-night singing & playing. That's when you get to meet new people and mellow out with some music. Most people that aren't joining in can sleep through it. Most kids sleep through it.
I'm surprised anyone would complain. Especially as this was going on inside a caravan, so can't have been all that loud!!!

Separate quiet & noisy areas are a good idea though.

KRis