Not sure I follow that part about parental controls not working.
The easiest way to limit internet access is typically to give them an account with parental controls on, and lock it down. Only give them access to Safari (not other browsers you may have) and then build them a white list.
Biggest limitation is that with a white list, you have to add every site they can go to.....a big pain, and never ending.
The only other way I know to make the web safe(er):
Probably others too.
Beyond that, you can use a Web content filtering service, but you may not want that for all computers all the time. Some routers come with it now (usually a paid optional service), or you sign up for one
Regardless......strongly consider not letting kids on your account. Besides their exposure to the web, they could destroy your data. Accidentally, or otherwise.