CovenantEyes is good at what it does, and probably the best in the field. It's filtering and tracking abilities are great for following a specific person around on the internet, and give you the ability to report back to the parents (or an accountability partner). It can feel creepy, for better and for worse, using image-detection tech to attempt to identify what you're looking at, and not just filtering internet access. This means you get some tracking of things like images sent via IM and whatnot. It works best on desktop and Android, and requires a bit of extra effort for iOS (like preventing using Safari and forcing the use of their browser for full effect.) It also doesn't work on Chromebooks. It's US$15.99/month.
The biggest flaw with CovenantEyes as a family tool is that it's tied to specific devices. A sneaky rebel could just borrow a friend's table and bring it home, connect it to wifi, and they'd have full access to whatever they want.
My recommendation is a service called CleanBrowsing.org, which works with a simple change on your router (nearly any router works!), causing all traffic on the network to go through their filtering system. You can also apply the settings to individual devices if they frequently are taken off your network (like for school and such). Because it's done through the network, it works on any device on your network, assuming they haven't overridden the settings on the device itself. (Easy to prevent with parental controls on the device.)
CleanBrowsing starts at free, with three generic options: security (just preventing access to sketchy/scammy sites), adult (blocking access to known "adult" sites), and family (blocking more mature content). The paid versions give more customisable options along with reporting features.
The two are different products that do different things, but I'd say CleanBrowsing is a better all-round option, and I wouldn't recommend CovenantEyes on it's own, rather, as an additional layer if there's, say, an extant known issue that you're wanting to address.