Kids ages two to fourteen spend $0.21 on digital content out of every dollar's worth of entertainment they buy today, according to a study in the NPD Group's new 'Kids and Entertainment Content, 2011 Edition.' That's up from just two years ago in 2009, when the ratio stood at $0.15 out of the dollar. And they start young; by the age of seven, 50 percent say they have paid for their first digital purchase—with and without help from mom and dad or their friends. "Kids are not skittish to use technology that is available to them," said the NPD’s analyst Anita Frazier.

Music unsurprisingly makes up the lion's share of the entertainment media kids buy online with a whopping 72 percent share. "The ease and short downloading times associated with music downloads means that this is often the gateway activity to downloading for kids," explained Frazier. Movies came in second, with videogames, television shows, books, and apps rounding out the list. For videogames, the rate at which kids buy digitally continues to grow, up 17 percent from the 2009 study.

As for how the kids buy the content, most of it gets purchased by the download. TV shows and movies are the exception, with subscription services accounting for just over half of the sales. That corresponds with the rapid expansion of online video rental services over the same time period.

The continued growth of digital distribution for entertainment media ranging from music to games won't surprise anyone in 2011. That kids are making those purchases so young, though, points out just how much a part of ordinary everyday life buying media online has become.