He never gives up. It's not the first time, I believe he did it for not less than 5 times.
This time, he removed the password for his Daddy's account which we have for access to our home PC. How did he do it? You may wonder.. Well, most of the time, I would leave my PC on while I do my other stuff at home. As our study room is just next to his room, it's not difficult for him to walk in and access the PC (which is left idle) to meddle with it while Mom is downstairs busy attending to other things.
Using Daddy's account which is obviously free from any parental control, he could be sneakily surfing (though it may just be for a few minutes) without Mom's knowledge while she is preoccupied with other things.
This may not be the best way to overcome his hacking but as a temporary measure, Mom has to keep reminding herself to LOG OFF before leaving the computer.
Lesson from the kids : Log off from the computer, there may be some hungry 'thieves' waiting to hack it. Do this only if you believe in parental control. Otherwise, don't even bother to read this post.