the reason to talk about bonds is that everything in finance is a promise and bonds are the most basic kind of promise 

even cash is kind of like a promise: it's a promise that you can redeem it for something of value in the future.  That understanding of money as a promise is a good entry way into understanding inflation and bitcoin

what is interest , why does it exist is a good one yes, do you think bonds might be too advanced for a 12 year old, or just have that as a later module ?

stack_harder

I think the most important thing is just to start with the basics:  what is an interest rate?  Why do borrowers have to pay an interest rate?

Then, what is a bond?  Are the buyers of bonds borrowers or lenders?  What about the issuer of a bond?    What would cause demand for bonds to go up/down?  What would cause supply of bonds to go up/down?  What does that do to interest rates?

AskSN

Education

econ

Best topics  for teaching kids about money and finance

I think the most important thing is just to start with the basics:  what is an interest rate?  Why do borrowers have to pay an interest rate?

Then, what is a bond?  Are the buyers of bonds borrowers or lenders?  What about the issuer of a bond?    What would cause demand for bonds to go up/down?  What would cause supply of bonds to go up/down?  What does that do to interest rates?