Or simply put the email address of the childs friend into MSN messenger and use exclusive chat

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2nd of all not ALL chat systems will agree to those rules or implement the system (many porn sites have no age check system). Then when the children ask for a credit number, the parents to tell them NO (because they don't understand the system), then the child will find the 'free' chat rooms.
All you have to do is tell the child not to share personal details, most children old enough to use the internet will understand. Then you occaisonally check them and see who they are talking, or you could also look at chat logs.
I imagine it would be very hard to prevent 'anyone' getting one of these id numbers.
If you put all the children in one centralised place, it will be very easy for people to target them.