I've been to the Leeds festival twice and I see a few people every year who take their kids. In honesty, i think it's a wonderful idea it, allows kids to have the experience from a very early age and in one sense I wish my parents had done it. However, it's totally dependent upon age, at between 0 and a year and a half old it's probably ok because they can't move very fast but hey proabaly won't appreciate it. between 4 and 9 it's most likely a pretty bad idea, because they can run, they do silly things and you can't have your eye on the the whole time. Once they're old enough to appreciate the experience, which is proabably around 10 (at the earliest), then it's proabbaly ok.
But look on it this way, your at a festival (which you've paid a lot of money to go and see), this is your holiday guys and it's a massive chance to see a tonne of your favourite bands in a very short space of time. Be selfish, leave the kids at home with grandma and you'll have a lot more fun. Take them when they're older and have more respect for of being in the middle of know where surrounded by thousands of people you've never met.
Stilgar, it was a lovely idea and I hope it went well. However I wouldn't have done, I think the kids are too young to appreciate it and your trading in your own enjoyment. Respect to you sir.