For the same reason that I try to brew my own coffee at home instead of purchasing expensive (and often higher in caffeine) coffee drinks while I'm out and about, I can't afford for my kids to get hooked on coffee at a young age. It's an expensive habit. If your kids do drink coffee, insisting that they pay for it may make a difference in the amount that they're able to consume on a regular basis.
我试着自己在家煮咖啡，代替出门在外或即将出门在外面买昂贵（通常高含咖啡因）的咖啡。同样的理由，我不能够让孩子从小就迷上咖啡。这是一个昂贵的习惯。如果你的孩子真的喝咖啡，跟他们强调，他们花在咖啡上的钱对他们每天的日常开支有很大影响。