I would go from basic like debit and credit , I bet it should be easy for them to understand the terms. Once they are familiar with the terms, they can learn about operating cost and expenses, depreciation, value of goods, profit and loss.

But the most important thing is a scope of business and the way they market it.

For retail, they don't require a much of knowledge. All they need to do is to document day to day transaction like Income in one column and Expenses in one column and by the end of they day they can just add it up.