There's also the fact that its 4 adults paying for all 9. And the cost of flights.

I've never thought of chartering as a cheap vacation. It can be very reasonable, but there's many more cheap alternatives. Cheapest trip I ever took was probably a week in the outer banks. House rental for 3 families was about $4k for the week and we drive there so travel was cheap. Food was similar to home, so no real added cost there.