Details like how and why pixels work are much easier to understand and retain if they're in some kind of context. Gamers learn about pixels to better understand the games or their gear. Devs learn about them in order to control them. Artists learn about them to better understand why computers mangle their art. So maybe choose an area the grandkids are engaged with and look for something there that also touches on display tech.
If you can zoom in enough to see subpixel elements, and pull up a color wheel, it's very intuitive to see that the screen is made of pixels, and the pixels are made of three elements.
https://en.wikipedia.org/wiki/RGB_color_model#/media/File:Ad...
The Pico-8 might be a approachable way to explore retrocomputing. It's an in-browser console modelled after GameBoy-era consoles.
https://news.ycombinator.com/item?id=18240375
I don't know about books, though. I remember reading the binary/computers chapter of How Things Work and being thoroughly confused at that age. There was some extended allegory about white mammoths and black mammoths.
Now I have both editions and have looked through them side-by-side. It's a bit unfortunate that some pages were dropped to make room for the expanded digital section in the newer edition, but well worth the trade-off for more/better explanations in the digital realm.
Just took a look over my bookshelf, but they haven't made it with me through moves.
I do still have Incredible Cross-Sections. That was another of my favorites to flip through. It looks like there are reprints and new versions. Would definitely recommend for kids.
https://www.amazon.com/Stephen-Biestys-Incredible-Cross-Sect...
It's not like modern display panels and HDMI signals and digital protocols are simple, but you don't have to go into the details to understand how pixels and framebuffers work.
If you're trying to explain how pixels work, going back to 1980s era technology is a detour into arcane technical details that aren't really relevant any more. They're pretty cool to read about if you're into retro computer technology but they don't have much educational value or relevance to the modern day.