Haven't tried taking them to see the opera as yet. They have been to see the Lion King musical, which they thoroughly enjoyed.
I'd probably also try light opera, such as Gilbert & Sullivan, but would be reluctant to take them to anything more serious as I don't want to subject my kids to too much frustration at a young age.
I remember going to see Mozart's "Don Giovanni" years back and whilst I enjoyed some of it, I was bored to tears by other parts. I particularly remember part of it where the Don's factotum was wondering whether he could get away with stealing a bit of food from the banquet table, and he spent about ten minutes going from one end of the table to the other and singing about "Should I take some? I'm so hungry, but I might caught! Oh, I don't know what I should do. Should I take some? No, I won't! Yes, I will! No, I won't - but I'm so hungry!" and so on, and on, and on.
About halfway through the song, I felt like shouting down at the stage, "Just take some of the goddamned food and get on with it, man!"
My dad always said that operas have two or three good songs, which everyone knows, but that the rest is largely just boring padding, and once I'd been to see a few operas, I had to agree with him.