If you argue at all about money (and most people do — at least sometimes), then there’s a good chance your children or grandchildren have already experienced some discussion of family finances. So, that’s all the more reason to make sure that they hear more than just arguing — that they have the experience of listening to calm and reasoned discussions of the family budget.
It’s not necessary for younger children to know every detail of your financial life, although they should gradually be included more and more, as they get older. But, they should know, in general, how things are going — whether times are good or not. I feel that these areas ought not to be a secret — they have a right to know, and sometimes they have a responsibility to know the ups and downs of the family they are part of.