A credit card with out them knowing the value of it should be a No No.
I would ask them to save some money like 250-500$ over a period of time from their pocket money or from doing a part time job. Then introduce them to something called "Secured Credit Card". Most Credit Unions provide it. Here's how it works, you deposit 500$ and you get a credit card for 500$ limit. Ask them (teach them) to use budgeting software like Mint.com to monitor their credit card (you should police them). Teach them budgeting and saving and paying off in time. Show them how they are building their credit score over time. You better follow all of this advice yourself before you preach.
If they screw up, their credit score screws up, and their hard earned/ saved 500$ is taken away by the credit union. But if they are disciplined they learn valuable lessons, there is nothing like it. When they get to college one of these credit card sharks is going to sign them in for a snickers bar or lousy t-shirt. Why not prepare them for it? Do you think they will not be introduced to credit card for ever? Instead be wise and teach them your self. It is better to lose 500$ in early teens rather than them having a 50000$ credit hole when they are 25.
I guess it is like sex education, it is better you teach them before they learn it.