The attempt to structure and communicate new ways of composing and hearing music has led to musical applications of set theory, abstract algebra and number theory. Music theorists sometimes use mathematics to understand music and mathematics is the basis of sound.

Peter Castine has written:

'Set theory objects' -- subject(s): Atonality, Computer composition, Serialism (Music), Musical analysis, Set theory, Data processing

I am not sure what music theory you are looking for. Music theory is very vast and required time and practice to know it completely.

You can check out some music theory lessons at pianomother and from there you can learn the basic up to the advance lessons for music theory.

You should look to find music theory lessons for your child by hiring a personal music teacher with a degree in music theory, or that have at least taken music theory classes.

Journal of Music Theory was created in 1957.