New OS for old PCs [closed]

As far as running Ubuntu on older hardware goes there are two main 'lightweight' version of Ubuntu: Lubuntu and Xubuntu. Lubuntu is the less demanding of the two, in terms of system requirements. Whichever flavour you choose, you should select an LTS (Long Term Support) version. The current LTS versions are 12.04, 14.04 and 16.04, with the latter being the most recent, and the version which I'd most recommend using.

When you install the operating system, the user that you create should be your 'Administrator' user, as this user will have the permissions to alter system settings. Once installation is complete, you can create your 'User' user without the privileges required to make far reaching system changes.

Be aware that the 'User' user will be able to save files on the system, and can alter their own user specific settings. You may want to set up guest logins if this behaviour is undesirable.

Warning
There is a risk when installing any OS, that it may not be compatible with the old hardware, and you may be left in a situation where you have no working OS, and nowhere to go to from there. Before installing either of the suggested versions, it is a great idea to select the 'Try Lubuntu' or 'Try Xubuntu' options on the installer. This should give you a reasonable idea as to whether your hardware can run the intended OS, before you commit, you could even use it to determine which flavour you prefer.

Personally I've found that Lubuntu seems to support a large range of 10+ year old PCs which previously ran on Windows XP.

Ubuntu and its variants are all easy to install. I'd recommend Ubuntu MATE for your computers. During the install process, create an admin user (with a strong password). Then, when you start the machine up, add a user with standard (or custom) privileges.

Ubuntu is excellent for user restrictions. Everything is locked down - users cannot install or remove programs, they can't delete critical system file and they can't change system wide settings. You can even limit their access to printers and external media.

As for the other things you require...

Web Browser:

Have a look at Vivaldi. It's a new browser with some really advanced features, that just works. It has an easy setup, and comes with Flash as part of it (the latest version), so you can play online games and watch videos.

Video Player:

Ubuntu MATE comes with a free and open source video player - VLC - installed by default. Just make sure you check the "Install non-free components" during the installation.

Photo viewer

A photo viewer - eye of MATE - is included by default. It can open pretty much all image files.

Office suite

Ubuntu comes with a free and open source office suite - libre office. However, compatibility with Word, Excel and PowerPoint is lacking and the GUI hasn't changed in 10 years.

I use WPS office on my computer - it looks very much like Microsoft office, and it will save as a .docx, .xlsx and .pptx very well.

My pronouns are He / Him