Here is a tip for helping your children build healthy, resilience-based paradigms about money:
Replace "We can't afford it."
This phrase, and the belief it expresses, sets up a paradigm of being powerless with regard to money. But if you frame your spending decisions as a choice, you take your power back.
Instead of saying, "We can't afford it," try, "I am not going to choose to spend my money on that today."
Sometimes, you actually can afford it; you simply don't want to spend your hard-earned money on the trivial or useless thing your children is asking you to buy.
If you say, "We can't afford it," you are wasting an opportunity to say something that proactively builds a better paradigm about the relationship between values and money. More on that in a bit ...
Even if you truly cannot afford it, saying that over and over to your children build a disempowered model in their heads of your family relationship with money. It communicates that you are never in a position to afford it and probably never will be.
When children believe their families cannot and will not be able to afford things, they will likely believe the same about themselves.
They could come to think their lack of money is a fixed trait and lose any hope of improving their financial situation.
Conversely: "I'm not going to spend money on that today" communicates that:
Framing it this way allows your children to consider what they value as well as how they might be savvy about the choices they make with money.
If your children have access to their own money, you can also say, "I'm not going to choose to buy this today, but if it is important to you, you can save your money [from allowances or birthdays] and buy it another time."