How do I protect my photos from accidental deletion when giving my phone to my kids? My little kids (less than 3 yo) like to view photos on my Samsung A5 cell phone. That's great, and they are allowed to do so. I can even pin the screen to make sure they can't leave the gallery app and start deleting e-mails, which is good.

Unfortunately, both the built-in Samsung Gallery app as well as the pre-installed Google Photos app allow them to (accidentally) edit and delete images, which is bad.

Can I somehow make those image viewers "child-safe" and turn off all editing features?

I also tried looking for alternatives, but, apparently, all highly-rated gallery apps have lots of "useful" features (like renaming and deleting files). There are also "kids mode" apps available, but they don't give access to all photos taken on the device and would require me to select the ones I want to show my kids (I want them to see them all).
Looking for a gallery app with Trash/Recycle Bin feature I came across Foto Gallery. If you delete a pic using trash icon, the pic is moved into trash (which itself is an album in this app). Within trash album you don't get the trash icon at all.
For your kid to delete a pic permanently, they would have to swipe leftwards from right edge, choose Delete now, and confirm to the dialog. I think that's tedious and provides enough safeguards from accidental deletion.
Furthermore, while the app does provide editing effects, the modifications are saved into the copy of an image, so the original remains intact.
Alternatively
You can continue with your default/favorite Gallery  app as long as your device is running Android 5.1 or above. Simply fire up adb and issue these commands:
adb shell appops set PKG_NAME WRITE_EXTERNAL_STORAGE deny
adb shell am force-stop PKG_NAME

Replace PKG_NAME with package name of your Gallery app. To know how to find out package see View app&#39;s full package name?
The first command denies your app the permission to write the external storage (also includes inbuilt internal storage), which means tapping onto the trash icon or using image effects would either do nothing or crash the app, depending upon how the developer programmed the app to handle such a situation. The other command force-stops the app so that new permission set applies afresh.
To reverse the changes, simply replace deny with allow in the first command.