If you're even a little bit computer literate, ignore the one star reviews. This is one of the most comprehensive add-ons that I've seen in Firefox. The bonus is that it also works on Chrome. Once updated, it works on Edge too! I used it for free for a long time before I realized that there are too many devices and too many browsers in my house to keep the updated list. I paid for it. I rarely pay for anything, so that's saying something. The cloud list periodically downloads itself back. It is obviously true that add-ons can be bypassed by a kid that knows how, but it's also true that Foxfilter lets you block the Firefox settings page and block it with a password. It is true that some of the default keywords are not very good (18, 21), but you can easily change them. I have my list published on 4bz.org, file name is FoxFilterList.zip

The best answer to keeping your kid safe is on the DNS of your router. Open DNS, Clean Browsing DNS, and others are good. You can use those for free on your router or on your phone service provider. For the keywords of the sites, Foxfilter is better. It's not really possible to name every website that might be needed, but getting the keywords in the title and meta data of the page does great. There's no perfect solution, but this one is good.