Linux

I've got a lot of experience in that domain, since I've upgraded/installed by helping 7-8 friends & family to switch to linux in the last year here in Greece.

So the two most important things here is the speed of the CPU, and the amount of RAM. With 4 GB RAM on both laptops, means you need to aim for XFCe or Cinnamon, not gnome/kde, and not generally heavy distros like ubuntu/fedora. Also, you need to instruct them to not open a gazillion browser tabs, they will hit the swap (and eventually crashes) with 4 GB of ram.

The Acer laptop scores only 600 points on the Passmark CPU test, which means that it's only good for XFCE. So I'd suggest the Linux Mint XFCE edition.

The HP laptop has 1400 points, which are plenty to run Cinnamon (the default Linux Mint edition). For comparison, most new laptops sold today have over 12,000 cpu points, some go to 30,000.

Mint is the easiest to update, and install new software, and it will provide a familiar look to the user. I highly suggest though a few changes done by you before you give them back their laptops (if you're the one making the installation):

[Cinnamon HP laptop]

• Install the Cinnamenu panel addon, to provide a more modern look to the main menu (and then modify it to look nice)
• Install the dconf-editor and disable tap-n-drag. This default behavior can drive mad Windows users.

[XFce Acer laptop]

• Modify the looks of the window manager to not have too many buttons, make it more windows-like.
• Unfortunately, tap-n-drag is not possible to be disabled on XFce

[for both laptops]

• Download Chrome. While Firefox is the preferred browser, Chrome is actually faster (particularly on youtube), and it consumes less RAM (tick its checkbox to consume less ram in the settings). This is seen as an anathema here, but the truth is, in lower end spec PCs, the speed difference between the two browsers is apparent.
• Setup their youtube to play at 480p by default, and disable autoplay. Anything else will be very taxing to the cpu.
• Install games from the repos for them, so they don't waste all their space with flatpaks later. Simple games like: sudo apt install aisleriot ltris gweled xye lbreakhouthd frozen-bubble gnome-mahjongg gnome-chess stockfish
• Second keyboard language if they require it
• Set up the power options to make sense
• Create a webapp launcher for Photopea (using the chrome option, as it's twice faster on photopea than firefox), so they have a photoshop clone easily accessible (gimp won't cut it).
• Set up the distro to be able to run appimages (test it with the new version of kdenlive for example from their website)
• Install OnlyOffice appimage and set it up in the menus. Onlyoffice provides better msoffice file compatibility than libreoffice.
• Install ublock origin or lite on the browsers, to avoid most ads and speed up the experience.