It's not that hard - 9 year olds aren't stupid. You just have to show them the change in pain a couple of times for them to "grok" it.

The pain, in Factorio terms, is having to sit there not doing anything. Boring. Once you set up an assembler to do it for you, suddenly you can fill that time with whatever you want to. The simple problem/solution/reward loop Factorio presents around automation works remarkably well.

Optimization, however, is harder to teach.