The idea of a pixel is that it's the smallest area of the screen that can be independently controlled, thus the designation "pixel" for "picture element".
In most modern phone screens or monitors, each pixel is formed by a group of three smaller elements with fixed colors (usually red, green, and blue) but _variable brightness_. By controlling the brightness of these sub-elements, we can control what the overall color of the pixel appears to be - once you get more than an inch or two away from the screen, the light from the element group blends into what we see as a single color - so 100% green + 100% red + 0% blue looks like bright yellow. 50% each for red, green, and blue looks like a middling grey. You can usually see the structure of the pixel with a magnifying glass, though this is easier with an old TV or monitor than a modern phone.
These pixels are laid out on a regular rectangular grid, and your display controller will offer some way to set the color of each pixel and to then update them all at some (usually) regular interval, for example 60 times per second. In computers, it's common to keep a "frame buffer" around that stores separate values for the red, green, and blue components of every pixel on the screen. If these are 8-bit values, that implies that each R/G/B component can have 256 different levels of brightness, and in combination they allow each pixel to take on one of about 16,000,000 possible colors. So, a program can change pixel colors by writing different values into this buffer and waiting for the updated buffer to be processed by the display controller to change what the display is showing.
Of course, the electronics, physics, chemistry, timing, and logic of display generation have changed quite a bit since Pong. And not all displays even have pixels - vector displays used to be a thing, and are still used in some very specialized applications.
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.
https://www.youtube.com/watch?v=niKblgZupOc - PC CGA
https://www.youtube.com/watch?v=_rsycfDliZU - Apple and Atari
There's also a linear algebra appendix [0] that presents the operations, explains how to use them, and how they can be interpreted, without going in any theoretical depth about why these things are the way they are.
[0] https://gabrielgambetta.com/computer-graphics-from-scratch/A...
On modern machines, a pixel is just a set of three numbers indicating how much red, blue and green light should be shown at a particular point. Ex: {0.75 red, 0.0 blue, 0.5 green} for a kinda-dark, orange pixel. The GPU keeps a big 2D grid of these number-triples in memory and on a regular schedule sends out a copy over the DVI cable to your monitor. The monitor has a bit of memory to hold it's copy. And, it has hardware to scan over the grid of numbers to produce a sequence of voltage levels that are used to change the color of the points on the LCD.
There's a bit of math involved in how to do a good job representing colors with numbers and how to convert those numbers to voltages. But, at the most basic level, an image is just a big 2D grid of numbers. If you want to change the image, poke the grid. People want to change images a whole lot. So, we've developed pretty sophisticated hardware and software around poking 2D grid... But, that's a whole other topic.