Singing is a communication art. Pitch requires maturity and control over the voice. Listening is the basis of all musical experiences. Sing simple tunes with your child. Nursery rhymes, camp songs and church hymns all have simple tunes.
Children breathe well naturally.
Children won't need this level of voice training until their teens. Learning music notation is much easier on an instrument. Piano and violin are excellent beginning instruments. Suzuki and Yamaha both offer courses for very young children to learn to read and perform music. Call your local college music department and your local music store for teacher references.