I buy my son the official playstation magazine each month.
http://www.myfavouritemagazines.co.uk/store/di splayitem.asp?sid=2125&id=14218&ts=ppc&CMP=KNC -ppc&HBX_PK=official+playstation+magazine&HBX_ OU=50
Each month it has a demo Blu Ray disk full of playable demos of PS3 games.
A recent one (December or January I think) had 25 playable PS3 demos.
That way they can try a game out first and see if they like it, or that it is suitable for their age.
It also has reviews of games so you can see if they are any good (they give them points out of 10)
Make sure you get the OFFICIAL Playstation magazine as that is the only one to have playable PS3 demos.
It is about �5.99 but when you think a single game can be �40 it can save you a lot of money.