Linux

(Warning - yes, I know they won't understand fully anything of the following, but they will understand some and will remember it's not magic.)

First, show them how to make a paper animation (quickly changing pictures, lots of paper and two pencils are enough, don't even need two pencils, but eh).

Second, show them how to make a paper computer (look it up, there are even ready books for children ; that is a bit more complex, you'll need to cut some for registers and the "windows" to indicate current values and you'll do the operations manually, and you'll need more turning pencils).

Third, find some book about microprocessor design - I'm serious, you just have to show them in it the pictures about what is a decoder and what is a datapath and ALU, and what are interrupts, and what are registers (program counter and two-three other ones, suppose), and explain how this relates to the paper computer. Not much more.

Then you tell them that a computer is just many microprocessors running their programs, some run small simple programs to control dedicated devices, and some run big long complex programs. After that you show them some of the devices - like hard drive, RAM, video, audio, network card, thingies on the board. And tell that they work with other devices, like keyboards and displays connected electrically. And tell that this looks like a city.

For 6 years old this is not so good (but just like people normally do with airplanes and trains, you still should try, just this shouldn't be your only try by far), but when I was 8-9 years old and wanted to learn, someone explaining step 3 to me would have helped.

Step 1 my dad had done, step 2 I think he did too, and it was in some book for preschool education I read, I didn't know it was sky cool back then. Step 3 is more of an encouragement when you can't quite mentally make the leap, from small elements which you know can be combined into complex things, to complex things themselves.

This is not an advice to teach a toddler computer design. Just like people don't teach toddlers railway design or civilian engineering or automotive or airplane design. They still tell them various things of how those work, and build models, so they don't have ideas from medieval bestiaries about these being magical monsters.