Check out from the library or Amazon: “My First Money Book: A Guide for Parents and Children to Saving, Spending, Sharing, and Investing Your Money” by Reggie Nelson
"Watch the pennies and the dollars will take care of themselves." - Ben Franklin
This works with money and with other areas: Be faithful in the little things and the bigger things will take care of themselves. For example, if you don't get comfortable with telling little lies then you'll never tell big lies.
Have a yearly “review” right before school year. Raise allowance as appropriate. If they want more money, they’ll have to take on more responsibilities.
Practice “going without.” Talk about how nobody gets what they want all the time. We should learn how to just choose to go without something we want. Get them in on the discussion and pick something every day to do without.
There’s always a temptation to throw money at your problem. But remember there’s nothing you can buy that will make up for what is really needed. And the stuff that accumulates while you try to fix the problem just ends up being a problem on its own.