I am very happy to read that you and the children had a nice Thanksgiving and were able to spend it w/family.
Now, let's visit the MIL situation. First and foremost, what popped into my head was...how in the world could you afford a trip to Florida and when you are having financial difficulties at this time. Yes, you want your children to visit their grandparents, but you continue to talk about your finances and trying to scrape funds together to pay your bills.
I certainly would not send the children down there by themselves and if you had the money and could make the trip, I would stay in the area and advise the in-laws that you are coming down and will be staying at a hotel (give the hotel name) and the dates. You could tell them that they are more than welcome to visit w/the children, but they will be spending their nights at the hotel.
As for your son having the impression that the in-laws can set visitation is a concern to me. I may be wrong, but he sounds like he's in the stage of rebelling against rules, etc. Of course, teenagers are like this, but I get this impression he's having a very difficult time understanding what a person in authority has the right to say or do when it comes to making decisions that are in the best interest for him and his sister. Do you sense that he doesn't respect you or your decisions?
Your son has missed a bit of time from school. As an incentive to get him to go, why not suggest a possible trip to FL in the spring or early summer if he goes to school every day unless he's absolutely sick? As a mom, I know you want to do everything possible to give them a happy home life, but you shouldn't reward either of them when they have exhibited bad behavior. Just my two cents.
A trip to Florida is a nice idea, but unless you find a goose that lays golden eggs, I do not see where you can pull out enough money to go down there for a few days at the moment. That's just my opinion from reading about your financial woes. Maybe a trip in the spring or early summer would be a better idea????
Sit quietly, the answers will reveal themselves when you least expect them to. The past is gone, the present is a gift and you need to focus on today, allow the future to reveal itself when it is ready.