A. Apple iTunes download service offers parental controls! Your child can buy an iTunes card at most stores and presumably use it without access to a credit card. Apple offers parents the choice of locking out certain categories of music with parental controls. This could be a nice compromise for many parents who want to allow their child to download music but want some censorship options.
To use the parental controls in iTunes, go to preferences and choose the 'parental' tab. Some of the options available are to disable podcasts, disable music store, and disable shared music. You can also choose to 'Restrict Explicit Content' and your child will lose the 'buy' button for these songs.