My kid started with computers around the same age. Using mostly text editors to type in random things on my computer. It was a great way to explore letters and learn to use a keyboard.
Following that we 'built' a computer together out of paper. This allowed me to show each part and what it did to give a base understanding. Then I let them hook up the RPi to a monitor and keyboard. The device was configured beforehand and had all the updates and software needed.
I chose not to connect it to the internet. That's how I learned computers and I thought it would be a good way for my kid to learn. Turned out really well and documentation is available on the device if you show them how to search for it.
As for educational software, I highly recommend GCompris[1]. It's just an awesome set of tools disguised as games. Accessible for a wide range of ages. I let my kid run wild in there and I'm blown away by the things it's helped teach them.
Last thing I'll mention is very specific to my interests, but I installed a few roguelikes. Easily accessible terminal games. Some well known like Nethack and others simple browser games I've made myself. All of them have gotten play time. In an age where graphics have overtaken good gameplay, I wanted to present games that weren't all lights and colors. Most of them are stripped down to learning the world and problem solving. Being turn based helps give time for them to think about each obstacle. And I feel like the constant death and resetting has helped alleviate early troubles they had with failure.
- [1] https://www.gcompris.net/index-en.html