This is what I would and would not do.

1. First ask yourself what they would like to get out of their book keeping exercise and ask them the same question. Based on that you should tailor your presentation.

2. It sounds like you are talking about little stores so I would focus on their core business processes,  main expense items - Cost of supply, Rental, their own wages etc. and main revenues which is basically the sales. What are the key question they have on their book regarding these items and how best to keep their books so that they get the answer with ease.

3. I would not use any Accounting Jargons.

4. I would give them a concept of financial standing at a given point (balance sheet - don't have to use that term) and an Income Statement. Give them an idea of what data they need to collect to be able to answer their business questions down the line.