Hi all... I had this problem too on my wife's mini iPad and here is how I managed to fix it.
After some trial and error, I did the get plist editing solution using iBackupBot tool available at http://www.icopybot.com/itunes-backup-manager.htm. A couple notes:
My backup contained two plists named Library/Preferences/com.apple.springboard.plist when I opened it in iBackupBot. (For ease in finding them, iBackupBot lets you click on the Path header to sort the files.) The second instance already contained
<key>SBParentalControlsBlockTimeIntervalSinceReferenceDate</key>
<real>362258969.244102</real>
<key>SBParentalControlsEnabled</key>
<true/>
<key>SBParentalControlsFailedAttempts</key>
<integer>9</integer>
<key>SBParentalControlsMCContentRestrictions</key>
<dict>
<key>countryCode</key>
<string>us</string>
</dict>
I made the highlighted changes below:
<key>SBParentalControlsBlockTimeIntervalSinceReferenceDate</key>
<real>362258969.244102</real>
<key>SBParentalControlsEnabled</key>
<false/>
<key>SBParentalControlsFailedAttempts</key>
<integer>0</integer>
<key>SBParentalControlsMCContentRestrictions</key>
<dict>
<key>countryCode</key>
<string>us</string>
</dict>
<key>SBParentalControlsPIN</key>
<string>1234</string>
I found that the only way I could make these changes actually transfer to my iPad during restore was to connect my phone to iTunes and wait for it to backup and sync. Then, while still connected to iTunes, I used iBackupBot to edit the backup file. WITHOUT ejecting my iPad, I then started the restore process based on the most recent backup. After the restore, I was able to use the above password (1234) to unlock my restrictions again. Hope this helps.
This worked fabulously well.