I just picked up a nature's goodness kefir box. It's got 5 sachets in it. I'm a little confused about it. I thought once you had your grains growing that you could keep culturing them forever. However, on the box it says that each sachet can only make about 5 litres? I'm confused. Does anyone have any tips for me please? I'd like to give it a go but have no idea what I'm doing. Is it good for kids to drink as well?