That would be a good idea to teach both of your cousin about bitcoin and the benefit of saving it. But it is not an assurance that they will learn how to save money, for me the best way to teach kids to save money is telling them real scenario about life of someone who doesn't have money and if they keep on spending then it is possible for them to be in that situation and I don't they want that to happen to their to them.
I agree with that, it would be so kind to teach his younger cousins how to saved money. But I also think that using bitcoin in teaching them will be too much. Bitcoin is too complicated for them to understand, saving bitcoin means understanding how valuable and how important saving it is. Normally, kids at that age would always want something or will want anything that catches their eyes but not all of them.
I believe that in order for the kids to learn how to saved money they must not just experience a real life scenario but they must witness how to saved money and see for themselves how saving money is very useful. In other words, a kid can learn saving money from his home, I think this is where it should start, one must see from his parents or family members how they saved money.