Junket is a sweet, creamy dessert. It's unlikely a family could sustain themselves on it, let alone live a whole life on it!
Perhaps you are thinking of a different dessert or a different story altogether.
Could you provide more details about the book you are thinking of? For example:
* What genre is it? (fantasy, children's fiction, etc.)
* What is the general plot of the story?
* Do you remember any specific details about the book?
With more information, I might be able to help you identify the book you're looking for.