deleted the "~/Library/Preferences/com.apple.systempreferences.plist" file, but that didn't work either.
Try it again but this time afterwards,
Disconnect all peripherals from your computer.
Boot from your install disc & run Repair Disk from the utility menu. To use the Install Mac OS X disc, insert the disc, and restart your computer while holding down the C key as it starts up.
Select your language.
Once on the desktop, select Utility in the menu bar.
Select Disk Utility.
Select the disk or volume in the list of disks and volumes, and then click First Aid.
Click Repair Disk.
Restart your computer when done.
Repair permissions after you reach the desktop-http://support.apple.com/kb/HT2963 and restart your computer. After you reach the desktop, empty the trash and try the parental controls again.
You have to login as an administrator, Then go to system preferences and click parental controls (it will prompt you re-enter your administrator PWSD again, then select the acct you wish to change. Then allow you to access the parental controlled acct. Make sure to re-lock the pane or it will not be secure.