Not going to provide an answer for you, unfortunately, but I have heard of similar issues.
I also use this feature for my kids' computers and have not had the same problem. Fortunately, it has worked as advertised. So, the good news is that it can work; the bad news is I am not sure how to fix your issue.
Maybe if there are parental control preferences (a .plist file, usually in a library's preferences folder--probably the user's library), those could be deleted and the machine will generate a new .plist file upon reboot. Sometimes corrupt preference files cause issues.
Good luck...