Most parents do not need to be convinced. What I have found is that most parents do, however, need to be encouraged. For a parent with little or no musical knowledge or background, teaching music can be an intimidating thing. Most have no idea where to begin. I have found that the best place to start is with the most basic question.
Music is a language.
It is an auditory form of communication. As homeschooling parents or non-homeschooling parents we teach our children all kinds of language skills with almost no conscious effort. This can be accomplished with music using the same pattern of learning that we see as children learn to speak. First by listening. Provide lots of music for your child to listen to.