parental controls do not work

We use both an iMac w/snow leopard and a mac mini with Lion. Whenever I configure parental controls to allow certain applications by checking the boxes, also the Dashboard widget (tea timer particularly to have kids keep track of time allowed), I have had issues with Firefox not working for them, although checked, as well as Safari which also was checked. The widget also checked would not allow it. All the apps they want to use has a warning sign that the administrator does not allow use (though it was previously allowed). Then when we enter the administrator password to allow use all the time, the same box keeps popping up despite using the correct login and password and never gets to the parental control window. So I check use just once and it allows it but never allows it to use always. It also would allow Safari after having to sign into parental controls again (even though it was checked in the first place) but it would not allow Firefox at all. There was a point in time (during earliest use) when it was working, and though the parental controls were thereafter modified, once you modify 1 user, it seems to uncheck some apps for the other so we have to recheck the other acct. Basically, parental controls do not work.