That depends in your cousins if they know that spending to much is bad then they will be fine cause bitcoin and altcoins are only a currency that is used to send payments. Users can do whatever they like into it but if you teach them that saving crypto currencies or altcoin will make them rich then they will probably save it. Just teach them the importance of saving especially in the world of crypto currency.
Kids learn from their elders, how they are saving and investing and the best place for their learning is bitcoin world. I mean if I invest in bitcoin and tell my son that I have bought some digital currency which is not only a digital coin business but also like a regular currency.
I can use my bitcoin anywhere in the world for purchasing anything. My kid will definitely take keen interest in bitcoin and to invest in bitcoin he will try to save money.