The Raspberry Pi is excellent for kids (and adults). There are tutorials for any topic (https://projects.raspberrypi.org/en). A lot of people are not aware that the Raspberry Pi magazines (MagPi) and books are free as PDFs. Or you can purchase printed books or magazines.

- Magazines: https://magpi.raspberrypi.com/issues

- Books: https://magpi.raspberrypi.com/books

Micro:bit

The micro:bit is an even smaller pocket computer for kids. The BBC was a foundation partner, but now a Educational Foundation:

https://microbit.org/