It is silly indeed to buy a Spirit ticket to go to Disney and spend money there, but otherwise why not show kids the value of frugality? And risk of spending a night in the airport can be an adventure…
by thy way, for absolute cheapest travel:
you can drive to the airport and buy Spirit tickets even cheaper without paying “online tax”. They convert part of the fare into the online tax to avoid paying other taxes on that part of the fare. With a good sized family, it pays off. If you ask nicely and mention kids, they can even assign seats for free.