Kids enjoy music. One of the most popular requests I get during the year is "can we listen to music?" while they have some free time to complete things. Pulling a monkey's paw I say "sure, but I pick" which works out to be hilarious for me. Traditionally you'd wander over to iTunes and fire up whatever you had, but to placate a mass audience, that requires a lot of purchases and doesn't really satisfy that whim you have to subject them The Backstreet Boys without having to explain to friends later why you have all these Backstreet Boys songs on your computer. Plus, even at 69¢ to $1.29 a song, building up a nice library for this sort of thing is expensive.
Enter Rdio. Rdio is one of many music streaming services that is all the rage these days. Similar ideas include Rhapsody, Napster, and new to the US Spotify. You pay a flat fee a month and any little song you think of can be played an unlimited number of times. Rdio does this through their website and computer-only access can be had for just $5/month. If you have a Mac, there's a client app you can download as well. If you feel like spending $10/month, you can get some personal use out of it by having access to anything and everything on your phone, a nice side perk.
So you can unleash this upon your children for very little money: