Just another thought.
The cost of a festival ticket + camping over a weekend is generally not far off of what you pay for an official campsite down toward the south coast.
So, even if your kids are just camping and hanging out with friends they're probably still getting reasonable value in comparison.
Its all relative - and festivals are mostly excellent value for money when you add up all the people you meet, the concerts you see, the campsite singing you do etc etc etc....
Just try comparing it to the ruinous cost of a cinema film + burgers & sweets & drinks - runs into a ton of money for family entertainment for about 2 hours. That's why we go to festivals - and not generally to the cinema!
Kris