What's the most effective way to reliably trigger and observe hard water scale buildup at a micro level?
You have to find out what kind of hardness is present in your water. Is it permanent hardness or temporary hardness? The latter goes away with boiling. 

The easiest and visual demo for children is to show the behavior of hard water with a soap bar. Soap when added to hard water does not lather very well. Watch the video here:

https://www.youtube.com/watch?v=jDgSohKVLio 

https://www.youtube.com/watch?v=bBYsfEHKYCk

As a control, you would need to buy distilled water. 

Does your electric kettle develop crusts with time?