Check out App Protector Pro in the market,
https://market.android.com/details?id=app-com.carrotapp.protectpronew
You can select any apps such as Market to prompt for a pin prior to running. It will prevent accidental downloading of apps.
I know this thread is way old but it still amazes me that Google has not corrected this oversight. My daughter is 3 1/2 and she regularly plays with my Xoom while we are in the car traveling. I have tons of apps on there for her. Some of them educational and some just for fun. The device is great at keeping her occupied and content while on long trips. The only thing that I could find to keep her from purchasing applications was to go to Google Checkout and delete all my Credit Card information. I did get an app that locks down other apps from launching without the proper PIN code and was able to add the Market application from being launched. Only issue I had after that was her hitting the adds inside the applications she uses. There is no way I know off to block those from then launching the market. Anyways I just deleted all my Credit Card information until they come up with a solution to this problem. Personally I don't think they see it as an issue and I doubt they will ever fix it. It is most likely providing a substantial amount of income for them. I mean you only have 15 minutes to return an app now so if a child does make a purchase your pretty much hosed as you don't find out that quickly.
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