Do Paid Chores Pay for Themselves?

(Stockbyte)

My son now travels three days a week, and my daughter-in-law has knee problems. What to do about such tasks as gardening, lawn mowing, leaf raking, etc.? They could hire a gardener; but their kids, now teenagers, are confronting scarcity: Their allowances no longer cover the things they want to buy—they have become economic people.
To solve both parental and offspring problems, the kids have offered to engage in household production in return for extra pay. The garden now looks better, leaves are raked more quickly and the lawn is mowed on time—and the kids have more spending money. I have no doubt that paying the kids is cheaper than hiring a gardener—cheaper than the market solution. Of course, my son could order the kids to do the tasks, but paying them is a nice way to give them spending money. I wonder, though: Does their pay of, say $10, represent a $10 increase in income? Or does my son cut back on the things he used to pay for and now makes the kids pay for themselves? If so, do teenagers understand this kind of fiscal substitution?