And what if they did have a serious underground baby sitting ring? I could see one teenager acting as a coordinator, taking a 10% referral cut from each deal. One such teenager, acting as the manager for all of his or her friends, handling schedules and such, could exceed the levels easily.
And if you don't think teenagers can handle business and money like that then you haven't heard about high school drug dealers.
But my point about babysitters was that they are an example of the cash economy. Other work often paid in cash is lawn care, snow removal, food trucks, computer repair, auto maintenance, handy-man work of all kinds.
A lot of time those jobs are done off the books and the cash is used for entertainment spending, like beer.