You tell the children very clearly that you don’t only save for a month, but for many, many years, and that investment conditions over four decades, if you want to save for your old age, change and that you have to consider not only what you can save over that long period but also your ability to save, i.e. your earning potential.
It is really important that you, as a parent, explain to your child that it is not about saving for a month – that’s then a question of pocket money and whether I’d sometimes like to go to the cinema, etc. – but that to provide for one’s old age you actually have to save for decades. Then the interest rates balance themselves out.