Re: Multiple Google Accounts In One Device (Android Market)
Maybe I misunderstood what you were asking. Currently, it's not possible to have multiple profiles on a device (i.e. two different log-in accounts, each with their own apps, wallpaper, permissions, etc.).
So whatever account you put on the device, the user of that device will have full access to it. In the case of apps, if you don't want to have to pay again for apps that you may have already purchased, you're going to have to use the same Google account that you originally purchased the apps under. If that's your primary Google account, then your child will have access to your GMail, Calendar, etc. I'm quite sure you don't want that.
Essentially, for now, Android is designed as a one-person-per-device system. Although some apps, like GMail can access more than one account, there are no provisions to segregate the accounts. Access to one means access to ALL.
Your alternative is to create an account for your child and not give them the password, and set up a PIN for app purchases in the Play Store app. You'll have to either only use free apps or re-pay for any previously-purchased apps under that new account.