Maybe this link will help Christie:
http://www.teachwithmovies.org/guides/pirates-of-penzance.html
I am not a huge Gilbert and Sullivan fan I prefer full blown opera, but this is a story of romance and the main themes cover responsibility and trust.
I doubt your 3 year old would get anything out of it, I think all musicals and operetta and opera are better for an older audience, because usually you have to figure out characters, plot and things as you go along. Perhaps you can find the story or a synopsis somewhere and preread it to the children, so they know what to expect – that would help. Anyway, see what this link suggests.
Hope it helps. Linda