@Mykittensmittens You'll be fine with a Chromebook. It can run (almost) anything that runs inside a web browser, so if your kids use Google Classroom, they'll be fine.
They do have very limited internal storage, so you won't be storing a lot of photos / videos etc on it, but you can connect a USB stick or external hard drive to it via USB and store files on there.
There's a list here of devices you can attach and filetypes it'll support
support.google.com/chromebook/answer/183093?hl=en-GB#zippy=%2Ccompatible-accessories-for-chromebook
Personally I've just bought a laptop for my daughter for her to do her schoolwork on, the only reason I didn't go with a Chromebook is that she enjoys gaming. Even with the laptop I won't be getting her Microsoft Office, as she uses the software through Google Classroom for her homework.
With regards to family link, you're correct that you can set up on a chromebook, you can either blanket block all adult sites or only allow access to certain sites that you whitelist.
And finally, with expiry dates. Pretty much every device these days comes with a limit on how long they'll support it for. Most phones for instance will stop getting updates a set number of years after its released, usually 2 or 3. They'll still work after that, but you won't get the latest updates. Chromebook updates work in basically the same way.
A normal laptop with Windows works slightly differently, in that you can get update to Windows for a much longer period. For instance, Windows 11 has just come out, but windows 10 will continue recieving updates until 2025. You can also upgrade from Windows 10 to Windows 11 for free (as long as your computer can handle it) so you don't really have to worry about security updates.
As your Chromebooks will recieve updates until 2026, I wouldn't worry too much about it. Your kids are likely to have needed to upgrade for other reasons by then anyway.
Hope the above helps, I'm in charge of buying hardware and software for work so know most of this stuff like the back of my hand, so feel free to @ me if you've any more questions