I completely agree with the cash idea. Having "real" money means that you can see it come and go and once in hand, it is harder to let go of. It becomes yours. The cards don't do this, digital shopping is even more removed from ownership - it is no wonder there is mounting debt.
For reference, you are screwed, because you have a boy! They are insane. However, I have the opposite issue with Smallsteps, where she is a little too concerned with things - a little too safe. It is good in some ways, but will become a detriment later in life unless she can learn to balance and evaluate well.