Time limit settings not working - iMac OS 10.8.5

Sorry to be a broken record - have not seen an answer pertaining to the iMac 10.8.5. I have two sons being protected by NF. They are able to stay on for much longer than their allocated time limit. I just downloaded the free version last week, so I am up to date. Can you help or point me in the right direction?

Thank you,

Angela