When I was a young teenager back in the dark ages (aka the 1980s), I babysat for 3 kids the oldest of whom was a bit younger than yours -- maybe 8, 5, and 3. I did this for weeks at a time every day (during the summer) for what seemed to me at the time like "all day" (it may have been less b/c the parents may have had things set up so one left for work later and one came home earlier, but it was certainly 6 hours or so/day, maybe more). Funnily enough I don't remember what we "did." I mean, the kids played, and I fixed them lunch and made sure no one got seriously injured (and may have put the youngest down for naps and such). We were at their home, which was just a "nice" suburban home in a "nice" (pleasant, large lots, low crime) neighborhood. I was happy with what I earned, but I'm sure it was a "pittance" as compared to other alternatives or the overall budget of the household I was sitting for.
I'm not sure how much this sort of thing happens nowadays, but it is feasible (and, I think, legal). I liked the kids I sat fine, but I certainly wasn't providing them with "quality programming" or education, I was just keeping an eye on things. This whole setup was about a mile from my own home, and I had access to my own mom (though she was at work, she was nearby and would have helped in an instant if needed -- she never was) as well as the parents of the kids I was sitting (ditto). I may also have had the numbers of neighbors, and, indeed, had school friends in the neighborhood where I was sitting (which was about a mile from my own, and walkable between the two).
Now a (working) mom myself but just to one kid, we have realized we get what we consider great value from local camps and we use them extensively (this in spite of the fact that my DH is RE). Most run around $200-$300/week (though we've found as low as $90 through the town parks & rec and pay as much as $400 for one that's in a sport my DS is really interested in). But we're fortunate to live in an area where there are lots of great options, and to be able to afford them (and of course it's much easier with just the one kid).