And they're kids. Kids don't typically comprehend the significance of romantic attraction between people, or concepts like marriage, until at least their teens. And even then are likely working off a very basic understanding of what a commited relationship involves.
Marriage to a nine-year-old might just mean being together forever. But being together forver is a much simpler thing in the mind of a nine-year-old, than in the mind of an adult.
They get teamed up in so much fanart. It's great.