Japanese child-targeted portal Kids goo conducted a survey on mobile phone use, with the results in What Japan Thinks. Bearing in mind all the problems with self-selected respondees to a survey that was only given to members of a particular portal, there are some interesting figures:

Q6: What functions do you use on your mobile phone? (Sample size=652, multiple answer)
Clock, alarm clock 79.9%
Listen to music 62.3%
Electronic wallet functions 7.7%
Camera 81.9%
Anti-crime buzzer 25.3%
Other 3.1%

Considering the age of the respondents it’s not surprising that not many kids used it for wallet functions, but it’s a pretty high percentage of kids listening to music on their mobiles.  About a fifth of parents set limits on the functions their child could use, a bit less on who the child could contact, while 45.9 percent of respondents said their parents had set no particular rules.