Gervase,
Ihave a 7.5 and an 11 year old, Wadebridge Folk Festival put a lot on for the kids, that August Bank Holiday weekend, and Our kids LOVED Bromyard Folk Festival (early September). Also great, from experience was Miskin (wales) at Easter and Dartmoor Folk Festival in August.
We met up with a couple at a festival sevearl years ago, who had kids the same age, and our kids and us got on very well, since then, we try to meet up at various festivals, the kids look forward to seeing their festival friends, and us too, and if we have to retire to the camp site early, we can start a sing of drink and tell jokes 'till the wee small hours without feeling like we are missing out on all the fun!
best of luck, hope you manage to get to some festivals that you AND the kids enjoy!
Cheers, Lucy