In the US, there's usually a reduced rate for kids under a certain age (such as 12), and often very young kids (under 5 or so) get in free. Is this not the case in the UK?

I agree with you that folk fests should provide a kids' program. But as for subsidizing childless people so they can have cheaper tickets (LOL!), consider it a trade-off since some of their tax money goes toward public-school education for other people's children (or is this not the case in the UK either?).