Android :: Is There A Secure Pass Code System To Buy Applications

OK maybe I'm being paranoid and behind the rest of the world who has already done this: I'm afraid of adding my credit card to my Google Checkout account! My main concern being a super intelligent user of my Android phone who will buy a million apps (potentially!) - my 5 year old son

I used to have a credit card on my account - but that's expired
Knowing that the card had expired, I thought I would test out the buying of an app
I was surprised that it only took a few pages of my clicking yes to buy the app
(The purchase didn't go through - I got an email saying this because my credit card couldn't be charged)

Question: is there a secure pass code system to buy applications?
Like needing to input a code at the last step of buying
(I think this is how iPhone works - I say this not having owned one)

I don't think there is a secure pass system - I've played around with my phone loads and have never come across anything. BUT, EVEN if there was such a pass system: there's still something wrong!
I've bought my phone. Not made any changes. Just started using. I would imagine that's what everyone does - as opposed to buying and then customising and making secure (and maybe optimizing the use of energy so that it doesn't die within a few hours of use)

Buying an application: you can cancel within 24 hours (I think that's the time limit) Over the weekend, I let go of business life and only pick up on Monday - so if my son purchased and installed an application on Friday evening, I wouldn't find out until some time later - because I don't look at my phone OR my emails over the weekend! (+ Say if an application was purchased that I then managed to cancel - I then lose the right to buy again and trial with the right to cancel - a small point, but valid regardless I think)

Would be interested to hear what others say - maybe I'm missing something and maybe I'm using incorrectly.