they charge a massive upgrade for any carry-on on checked baggage. Checking bags often kills the discount. You can use a personal item. I would check the most recent dimensions allowed for personal items and see how much you can work with a bag that size. They actually have small duffles on amazon that are made to fit the exact dimensions. For a family it might make sense to see if you can fit your clothes and stuff in one suitcase.

The seats are about what you would find on a commuter train. They have minimal padding, do not recline, and they have little leg room.

They have wifi on some flights, but you will pay for it.

I have found tickets on spirit that are so cheap, dealing with all the above is no big deal. But consider the distance. I have no problem flying spirit for an hour or two. But a five hour flight would definitely be uncomfortable and it would test my wife's patience.