[johnisgood]

I do not think unauthorised transactions are much of an issue, and as an other commented asked: what would be the alternative? Come on now.

The solution (that many people do not want to hear) is very simple: supervise your kid, be a parent.

Edit: I love getting down-voted for saying "be a parent". Laughable. We are on HN and you do not know how to prevent this from occurring? Read the other comments, there are lots of suggestions. You have absolutely no excuse.

[dotancohen]

Or just log out of your accounts when your not using them. Seriously, I do believe that I'm the only person I know who uses the Logout functionality of websites.

Or let your kids use a different user account on the computer.

[johnisgood]

There are so many ways to prevent this, we would probably be here all day if we started listing them. There is a thread with such ways.