I believe that parents should set boundaries and guidelines for their kids, and that they should frequently check up on them. But teens are just that, and even the best of kids can and do lie to their parents, and they can look at things and either get rid of the evidence, or know that their parents are not going to understand the device enough to be able to figure it out.
Safe eyes is one of the hardest screeners to get rid of. You have user names and passwords for the user, and then a seperate one for the Administrator. You can get instant alerts when the user has attempted to access something that is not allowed, and it is difficult to uninstall.