The way to a kid's heart maybe less through music at first, but more through stories. Here's a list of some of my favorite books about opera, suitable for kids. This is by no means exhaustive, just a sampling. Feel free to send in your own favorite titles:
Sing Me a Story, the Metropolitan Opera Book of Opera Stories for Children by Jane Rosenberg
Suitable for grades 3-6; and beautifully illustrated. Breaks down the intricacies (impossibilities!) of some opera plots.