The process to download and install the Kano OS is very well documented on the actual Kano website. I recommend just buying their setup. It is not that much more expensive than DIY, but the kit and the kit documentation is much more kid friendly.
You are not going to find a better kid sized keyboard than you will from Kano.
I used my own because I wanted a Pi for a NAS setup and this give me the opportunity to use it for my own stuff later.
First thing you will need to do is to get a micro SD card, a computer adaptor and download two programs,
Next, insert the micro SD into the adaptor and plug the adaptor into your computer
Open the etcher software and follow the prompts.
The image you are using is the Kano OS you just downloaded
Make sure the only SD or memory device you have plugged in is the one you want to install the Kano OS on, it will permanently wipe the drive selected.
Once selected hit the flash button to get started – it takes a while, this is a big program on a small card
Once its done, close it out and insert in your device to get started with your kid built computer.