Do a little research on the history of the waltz. Remember the children are inquisitive and will be more interested the more information you have about the waltz.
Show the child, depending on age appropriateness, a film that showcases this waltz style. Many Disney movies have a waltz scene between 2 main characters, like in the "Beauty and The Beast." Show the child how fun twirling around on a ballroom floor can be to peak interest.
Watch other individuals perform the waltz. Go to a ballroom dancing school with your partner or a friend and watch the waltz performed by others. Then take a few basic lessons on the steps so you can be familiarized before teaching the child.
Sit down with the child and pick a song that you will learn to waltz to with her. Let her have a large part of the decision making of the song, and remember that many songs will work as a waltz as long as it is in 3/4 time.
Teach the child to count the waltz counts (1-2-3, 1-2-3), since it will help him ingrain the rhythm. Ensure that the child understands the rise and fall technique that is unique to the waltz.