This is a great lesson for the whole family!
Give you children some options using your actual household debts. Feel free to take away a few digits if the numbers are too big for them to add or subtract.
For example, you could tell them you borrowed $25,000 from the bank to buy your house. You now owe the bank $10,000 because you have been paying the bank $1,000 per month for the last 20 years. If Santa brought you $10,000 for Christmas would your kids rather pay off the mortgage or use the $10,000 for something else. It could be education, starting a business (I know a lot of teens who would use it to become rich playing games on youtube), buying stocks, buying another house that people will pay you to live in, or anything else that comes to mind. They may have some interesting ideas of their own!
If they decide to pay off the mortgage, what would they spend the extra $1,000 per month on?
Then figure out how much the $10,000 invested could change their lives. An investment in education could make the difference between making $30K per year or $300K per year. Putting $10,000 in the stock market could double in 8 years. You get the idea.
The math does not have to be perfect. This is to get them thinking about making decisions that will affect them in the long-term.