Summer Sundae Weekender. It's been nominated a couple of times for Most Family Friendly festival, and has recently won eFestivals' Best Small Festival Award. It's very chilled out and relaxed.
Family campers are offered a 'family friendly' part of the campsite, and the children's area is like a little festival of its own - lots of play activities and entertainment. More importantly, it's on the opposite side of the building to the main stage, so less chance of damage to young ears. Also a good place for mum and dad to chill out (you're not allowed to leave your kids there without a parent/carer being present).
The site is secure (although relaxed) and the festival's maximum capacity is 6000, so it will never become too big:
http://www.summersundae.com/
There's a dedicated forum at the eFestivals website if you want some impartial opinions:
http://www.efestivals.co.uk/forums/index.php?s =f914fc84144fa4a54d3d89e0778d1f05&showforum=42
There's also Big Session - same venue, same chilled atmosphere, same kids' team, but very folk orientated. It's partly organised by Oysterband, so that gives you some idea of what the acts are like:
http://www.demontforthall.co.uk/bigsession/