OP: Kano looks nice, but it is just a Raspberry Pi 2 bundled with some extra bits and a different OS (which you can freely download for any Pi).
The ‘standard’ Raspberry Pi OS is Raspbian, which is basically a little Linux OS (similar to Debian or Ubuntu if you have used those?)
The Kano OS looks like it is a bit more kiddie friendly and comes with stickers etc.
Both come installed with a Blockly-style programming language that serves as a common introduction to programming.
I doubt either are fully usable by an 8 year old without parental guidance, but Kano is probably the simpler of the two.
You know your own and your kids level of tech-saviness – so I’d choose according to that.
As others have said, if they outgrow the Kano system you can turn it into a ‘normal’ Raspberry Pi with Raspbian just by putting in a different SD card.