Given that DD1 doesn't know any 'bad words' at all yet AFAIK, would I regret taking her to Billy Elliot? A friend's in it and his 6yo's been twice, but then of course you would turn a blind eye if you were in it wouldn't you - I'd do the same rather than keep her away from a show her daddy was in.
I'm a bit of a swearer myself and it doesn't bother me but I gave it up (somehow) when I had kids and have no desire to hear 'language' coming from the DDs' mouths before I have to.
I know this is a bit late after the original post, but we have just seen Billy Elliot in London and I thought I'd just reiterate the previous comment that there is a LOT of swearing in this show. The only word they don't say is the 'c' word. Other than that it's pretty much constant. I went with an 8 year old and 12 year old, and with hindsight I don't think it was suitable for the 8 year old.