Ok so I’ll just assume that this is the first time the kids will have a broader sort of contact with the online world and all its great and bad sides.
Also: some of the things I talk about might not apply. Just disregard them. This post is mainly meant to make you look at the topic from a different point of view.
I’ll be radical and say:
I know the Internet is full of dark and scary places (I’ve been there when I was a kid myself, despite all the attempts at filters) and if the kids are, like, 12 (I don’t remember their age, sorry. Time flies by) chances are pretty decent they know that and/or have been there already.
Us old guys (yeah I know you are 10 years or so older than me, bear with me for a moment) might think that it is becoming worse, but I am not sure we are right there. There have always been things like “Adolf Hitlers hate pages”, rotten dot com, the terrorist’s handbook, porn sites, chats infested by pedophiles and the like.
From my point of view filters make the problem worse, not better. They cause frustration (both for you and the kids) because they often block things they shouldn’t, and often do not block things they should. Many of them are rather easy to circumvent (kids are not dumb) and they also send the kids the message that you don’t trust them.
Probably you should trust the kids (to a certain degree that is). Some thoughts on that:
You may or may not tell them that you want to trust them or that you do trust them to a certain degree.
You may or may not tell them that you will be monitoring their traffic on a regular basis.
You may or may not install a monitoring program and you may or may not actually look at the results.
You should probably tell them that you expect them to work with those PCs, and only to work with them.
you should probably think about/define what consequences there are if they break the rules. But keep in mind that (depending on the situation) you should not overdo it, see below.
Quite important IMO: Make absolutely clear that there is nothing too embarrassing to talk about it. They have to tell you if there is something wrong, if they did something wrong that might get them or you into trouble, or if it causes discomfort for them.
Cyber-grooming, bullying, phishing, disturbing websites and fake news all exist, and everyone (including you or me) can fall for some of that ■■■■ and have a need to talk about it.
Some of it may happen even in the spaces you think about as safe. Just an example: some guy guessed a link of a video conference software used by schools in Norway and made a naked appearance in front of the virtual classroom.
Make clear that you are on their side if something goes wrong, and they do not have to hide it from you. Even if they caused it by breaking your rules.