Disclaimer. This is advice that is intended to be helpful. You may find it helpful. If not, remember it was free. Kudos are solicited. Complaints are not.
The first thing to note is- there is no perfect system. We are basically talking about security here and I can assure you that the smartest people at the highest levels have not achieved perfect security. Make no mistake, in the high level security circles as well as personal information security, it is a classic example of an arms race. At the top level, whatever the security people do to batten down the hatches, the bad guys are testing that system and looking for ways around it. Now, I am a parent and a grandparent and I love them all. However, if you imagine that children/teenagers are not testing the systems and trying to find ways around them, you are probably not a parent. If you doubt my assertion, I invite you to open Youtube and perform a search for how to bypass parental controls. I guarantee you will be presented with any number of instructive video choices. Kids make these for other kids. Welcome to a future where, unless you were a computer science major, you very likely know less than your children.
This leads my to my first security measure. It has two parts. One, talk frankly with your child about online dangers and things you do and do not approve of them viewing. Part two, as the parent of a minor child and the provider of things like computers, cell phones etc., parents have a right to access those devices and view what the child has been doing. Children need to understand that the parent is responsible for their behavior and safety and they will enjoy only a limited privacy. Children should provide passwords for all their devices, including smart phones and tablets, to parents. Each parent will approach things as they see fit. I told mine that I had no interest in reading every tweet, text, etc. but I would perform a general scan from time to time. Meaning I would scroll through and look for key words, inappropriate images etc. Often the understanding that a parent can at any time access the device is sufficient.
This is one of the most important pieces of the puzzle. The parent should have the only administrator account on the computer. The child should have a restricted account. Now, here is the key. When the parent is not using the computer he/she needs to log out of their administrator account and the password for that account should never be known to the child!!!! Most of us simply walk away when we are done. Get in the habit of logging off. At minimum, you can set your computer to sleep after a couple of minutes and require your password to wake up. If a child gains access to an administrator account, the child can easily bypass all your efforts. Whatever you think the child knows about computers, you are probably wrong. It is very likely that the child not only knows more than you, but knows more than you think he/she knows!
Regarding a Mac desktop or laptop to which your child has access. First look at this link: https://support.apple.com/kb/PH18571?locale=en_US
This will provide the basics. There are software products you can buy for Mac and PC. NetNanny seems to hold the lead as far as price and features.
If a parent has serious concerns that the child may be doing something dangerous or illegal, you may consider a keystroke logger program. I won’t get into the whole thing here. Basically it allows the administrator to know what someone has been up to even if they erase files. My suggestion is, this is a last resort.
If you have further concerns or need Mac help, I can be reached at: firstname.lastname@example.org