I think it's a great idea, with the caveat that unless the performance is presented as family or kid-friendly you might want to check out the content of the opera first (and its length!). Some of the themes are not suitable for all ages, and even if in another language a sharp kid will get the drift and could be upset.
I think it might help my son engage to know the story first before attending, and maybe even listen to parts of the music, so he could recognize parts he knew.
DS5 and I sing made-up-on-the-fly operatic style songs together sometimes, maybe you can join in to keep from laughing!
