There has been much research about music instruction and its effect on learning. I have been reading and studying Music with the Brain in Mind by Eric Jensen. The following are excerpts from the book.
"From five to nine is a good age to begin music lessons. Every child can play an instrument. Singing is still beneficial at this age. Read poetry that is highly rhythmical like that of Jack Prelutsky and Tom Glazer. Keyboard, piano, violin, or recorders are all great instuments for early music training. Children are also ready, as they approach eight and nine, to try their hand at composing simple songs. A handy guidebook on specific music selections is Good Music, Brighter Children by Sharlene Habermeyer. A good CD to purchase would be one that includes a vaiety of musical genres. This is the time to introduce reggae, classical, romantic, hip-hop , and pop. Soundtracks to Disney movies, especially the older ones, provide exposure to classical music, romantic music, and show tunes."