What sort of music do you like?
The secret garden party is fun and very family friendly, and quite eclectic.
http://www.secretgardenparty.com/2011/#/308/277/Fun line up this year, real mix of music. There's a kids camp at secret garden which from wiki - "Djs, storytelling, theatre, comedy and art installations, and all manner of wild activities, from snail racing to Space Hopper races and a children’s disco. The Kids Camp, which grows every year, has creative workshops, a ‘doodle den’, a ‘Young Revolutionary's Parade’ with home-made costumes, a fashion show (ditto), pizza baking, and the ‘Hush Hush Hideaway’, where over-stimulated little ‘uns can listen to music, read books or watch cartoons over a soothing mug of cocoa. Special kids-only events include early-morning adventure walks and an adventure aboard a pirate ship.
If all the mayhem proves too much for you, head for the massage and healing space, dance classes, yoga sessions, saunas and hot tubs (a nanny service is provided for an extra fee). You can camp in tents, campervans or caravans, or book tipis or safari tents. The family camping area, set away from the loud music and late-night revelry, features campfire storytelling sessions."
I've always really liked Latitude -
http://www.latitudefestival.co.uk/2011/Some nice bands this year and very family friendly.