One thing I think should be taught, as early as grade school, is what taxes are and what they’re used for.
A child should know her local sales tax rate and know how to calculate it and apply it to a purchase. She should also know about local postal rates and how much it costs to send a package somewhere.
A parent could set up a child with a simple savings account and teach them about interest rates and how they cause savings to grow (yeah, I know interest rates on savings are almost nonexistent these days, far behind the rate of inflation).
While we’re at it: teach them about inflation, and the present value of money.
I think these things would be a good start.
Or: building a playhouse. Even if the kids are using scrap cardboard, take a ruler or tape measure and help them figure out dimensions and square footage.
I used to "tax" my kids Halloween hauls. It made them realize that it's basically theft.