I found these rather suspicious looking errors in the system log. They appear whenever I try to make any changes to parental controls (in this case it was changing the allowed apps). This may not be a problem with parental controls per se, but rather with some missing code signing keys in the key chain.
stefan@libbys-macbook:Library $ tail -f /var/log/system.log | grep parentalcontrolsd
Jul 4 11:28:56 libbys-macbook parentalcontrolsd[924]: CreateTempKeychain [1174] -- mkstemp created path: /var/db/com.apple.parentalcontrols.keychain.9Frwdw
Jul 4 11:28:56 libbys-macbook parentalcontrolsd[924]: FCCreateTempKeychainForCodesigning [705] -- using keychain: /private/var/db/com.apple.parentalcontrols.keychain.9Frwdw
Jul 4 11:28:58 libbys-macbook parentalcontrolsd[924]: CreateIdentityWithKeychain [994] -- *** CopySigningIdentityFromKeychain returned NULL
Jul 4 11:28:58 libbys-macbook parentalcontrolsd[924]: FCCreateTempKeychainForCodesigning [708] -- *** FCCreateTempKeychainForCodesigning: CreateIdentityWithKeychain returned NULL
Jul 4 11:28:58 libbys-macbook parentalcontrolsd[924]: -[AppWhitelistHelper _processBundleIDs:forUser:kickMCX:] [286:] -- *** FCCreateTempKeychainForCodesigning returned NULL
Jul 4 11:28:58 libbys-macbook parentalcontrolsd[924]: FCCreateAppSignature [857] -- *** SecCodeSignerAddSignature returned: -67062 on pass: 1
Jul 4 11:28:58 libbys-macbook parentalcontrolsd[924]: FCCreateAppSignature [857] -- *** SecCodeSignerAddSignature returned: -67062 on pass: 2
Jul 4 11:28:59 libbys-macbook parentalcontrolsd[924]: FCCreateAppSignature [857] -- *** SecCodeSignerAddSignature returned: -67062 on pass: 1
Jul 4 11:28:59 libbys-macbook parentalcontrolsd[924]: multibyte ASN1 identifiers are not supported.
Jul 4 11:28:59 libbys-macbook parentalcontrolsd[924]: FCCreateAppSignature [857] -- *** SecCodeSignerAddSignature returned: -67062 on pass: 1
Jul 4 11:28:59 libbys-macbook parentalcontrolsd[924]: FCCreateAppSignature [857] -- *** SecCodeSignerAddSignature returned: -67062 on pass: 2
Jul 4 11:29:00 libbys-macbook parentalcontrolsd[924]: FCCreateAppSignature [857] -- *** SecCodeSignerAddSignature returned: -67062 on pass: 1
Jul 4 11:29:00 libbys-macbook parentalcontrolsd[924]: ### Attempt to send message without connection!
Jul 4 11:29:01 libbys-macbook parentalcontrolsd[924]: FCCreateAppSignature [857] -- *** SecCodeSignerAddSignature returned: -67062 on pass: 1