Simple: build a calculator. They are adults and should know basic math so they have a comfort zone and would know if the computer did it right.

Introduce loops (add a string of integers like "1 2 5 8"), functions to run those (add, multiply, etc.)

Don't get all computer science-y on them with recursion, sorting or stuff like that in the first 2 hours of an introduction.

It might be a lot of work to setup 20 computers with something like Ruby or Python - assuming you ask them to bring their own laptop. That might take 2 hours itself. Just find some online Javascript (shudder) editor thing which will allow them to build a basic calculator - no UI though, that's too complex for 2 hours.

Programming is just telling a computer to do things. They are parents and should be familiar with that concept, except the computer will do exactly what they ask, unlike kids.