Distro suggestions for grandparents?

Mint or debian will do the trick. Set up a script to update the system and turn it into a button or desktop shortcut, name it something like "click here to update" and done

Mint easy to install easy to navigate

I would go Debian, purely because it will result in less maintenance on your end. Maybe show them several DEs and see what looks easiest to them.

Red Hat fuckery aside, I still feel like Fedora is the best refined distro out of the box for the average user, and Gnome is the most consistent desktop. Immutability is perfect for grandparent types who don't need much of any customization, so I'd strongly consider Silverblue. Just make it a habit to go upgrade releases every 6 months, you should be visiting more anyway ;)

Make sure you put the internets on it so gram gram can whip some ass in online poker

I hate to say it but they'd probably be better off with an android tablet or an iPad.

Mint is the way to go. Easy UI, stupidly stable, relatively up-to-date, easy to fix problems.

I had some issues with an old HP Laptop not so long ago because it uses some weird Wifi that needs Bluetooth enabled and it was a hassle to get it working. In Mint it simply worked out of the box, so I stuck with it on that device. Mint with Cinnamon is also very similar looking to Windows and should provide a smooth transition for your grandparents.

I've been using Rocky Linux for awhile. As an OS, its pretty good. The only problem is software availability. Some fedora apps work. It also has flatpak. But for the gnome apps that aren't available in repos, I normally have to go and compile them myself if I want a more standard look and feel. Otherwise, it's reasonably secure, stable, and easy to use out of the box. I use it for mine, my SOs, and my kids computers, one of which is neurodivergent. I see it as the best possible canvas to build on. And, for me, its install once, cry once. The EOL is likely gonna be after the computer its installed on has long been obsolete.

I installed hacky ChromeOS to my grandma's laptop a few years ago. She never had issues with the OS itself, rather hardware issues. It's easy to learn and ChromeOS is literally just a web browser, it's not even useful for anything else. Though I would recommend you install something like Fedora Silverblue, Kinoite, or a distro from UniversalBlue instead. Container based distros are secure and hard to break.

mine are on bazzite since I already had an image on usb. since I did the install and included the ancient chinese windows games they like via bottles/wine (kept on a usb taped inside the computer since windows xp days), they have asked for nothing. I also set up their email and printer and some ancient obsolete crap they refuse to get rid of but no problems so far.

For grannies, I like to go with Endless OS. Curiously, it combines aspects of the two most named distros under this post; based on Debian and utilizing OSTree (like Fedora Atomic does).

It's often overlooked (for some reason), but actually combines the best of both worlds:

• Over two years of support (since release), while Fedora Atomic only offers 13 months of support since release
• Automatic updates are enabled by default and updates are applied atomically in the background, while Debian(-based) are not capable of atomic updates
• Does not even offer installing software through apt and doesn't even have it's own rpm-ostree counterpart. Instead, it goes all-in on Flatpak.

The only thing that might give something like Fedora Atomic an edge would be by installing any of the opinionated uBlue images (like Aurora/Bazzite/Bluefin etc.) that just apply and ship fixes for you (without requiring you to do anything for it) and that are even capable of automatically applying updates to major releases for you in the background. This is basically just hands-off mode.

I know how this sounds, but (rolling) openSUSE.

I have my parents and granparents on Tumbleweed, but I update it manually when I visit.

I set them with a big macos style dock at the bottom of the screen and that's all they need tbh (Firefox, Kmail, Office, Signal, photo gallery ... that's it).
Never had any issues, but can't complain much about Debian & Fedora either (I had them on Debian for like a decade, less than a year on Fedora, I think now 4 years on Tumbleweed), it's just that it was a pain for me to upgrade them when the time came, so I wanted a rolling distro.

However, if I ever have to set them up with a new PC (instead of constantly upgrading their rigs with old parts from my PCs), I'll try opensuse.org/Aeon. Seems low maintenance & low extra config for my usecase.

Just keep them on Windows 10 unless there is an issue. Don't make them learn anything else.

NixOS is an immutable file system if they are disaster prone. You will have to load everything for them ahead of time, browser, email client, Only/Libre Office, etc.

I had to switch a friend's mom to this because after switching her to Linux Mint previously, she somehow deleted the UI entirely.