Hard drive encryption is one method of denying access to a specific drive or partition on a drive. Businesses generally use a protection software to secure main drives where updates and changes are made from a central location. For the average user Windows provides the Group Policy feature. That mainly will be applied to XP Pro and Windows Server 2003. http://www.microsoft.com/resources/...roddocs/en-us/customize_with_gp.mspx?mfr=true
I know the feeling about "little fingers" touching things as soon as you walk away from the system. One freeware that may not specifically block a second drive but various programs and Windows itself is called WinGuard Pro 2006 and is a freeware. You can download that from http://www.dirfile.com/windowsguard_pro_2006_free_edition.htm
If your children have separate accounts, just make sure the second drive is using NTFS and set write permissions for their accounts to deny. Make sure you pick the option to let the permissions propagate to new files.
The Hide & Protect program there looked far easier to use then toying around with user and system settings. The creation of a separate user with limited access may not be what you would want when it was either too limited or not effective in the way you intend. Another way to hide a different drive is to actually hide the one or more partitions on a drive with a Linux partitioning tool.
You would set the main partition there to inactive where it would hide it from view in Windows. But you would have to boot from a prepared disk to make active again everytime you wanted to access it. That would soon prove to be a hassle.