How can I disable the Android Market temporarily? My kids play games on my tablet, and I would like to disable the Android Market and in-app purchasing for the duration of their use of the device. 

Is there any option, third-party software or any other way to stop my kids from buying stuff inside their games or installing any software?

I understand that I can remove credit card info from the account, but since that device is used by me most of the time, this is too cumbersome. I also don't want to stop Internet connectivity since some games are Flash games in the browser.
In the Market settings you can add a PIN to protect purchases; that should lock them out effectively enough.  I don't know of anything offhand to prevent all installs, though.  I would assume you'd need to be rooted for an app to intercept and prevent install attempts.

If you are rooted, you could use something like Titanium Backup to back and uninstall the Market, then reinstall it when your kids are done.  That might be cumbersome as well though.