I wouldnt leave the other kids behind as the default. I agree with other posters who suspect some (all?) of this is coming from the mother. But, excluding them outright will just foster resentment and give ammunition to what seems to be a festering problem.
If she wont play ball and you cant get a temporary amendment to the parenting plan through the courts, I would at minimum take out the trip insurance for the step kids portion of the trip and get written and notorized authorization from the mother.
Then you make it perfectly clear to ALL the kids that the trip is for the whole family, and it will be difficult for Mom to pull out...but if she does...well...at least you are financially and "morally" covered.
That said...and this is out of the box...if part of the Moms frustration is "better parent" syndrome, for lack of a better term...meaning she feels Daddy gets to have all the fun while she gets the brunt of the school days and has to play disciplinarian (I dont know if thats her perspective, but from what you described, I think it could be part of it...
Have you thought about inviting Mom along for a portion of the trip? Not the whole trip, but just fly her in for a day or two towards the end so she can take the 6 and 8 year old around herself and build a "shared experience"?
Every separated couple is a bit different, but I did this one year with my ex right before she got married. The kiddo and I stayed for two weeks, and her mom and 10 year old uncle flew in for the last four days.
Separate rooms...but we got along well enough that we did stay in adjoining ones.
We had meals as a family at dinner, but otherwise split up most days. It wasnt comfortable for me at times (by the end of the second day I recalled exactly why I liked my ex from a distance, but not up close and personal)...but it helped the kiddo understand that it wasnt a mommy vs daddy thing, and they had a blast doing stuff on their own a few days while I took the 10 year old uncle off on my own to do "guy rides".
Likewise, something like that would allow you two to spend time alone with the 5 year old without the 6-8 year olds feeling left out or on the back burner.
I even schemed with my ex to set up a big "reveal meet up" under the castle right before Wishes, and we didnt tell the kiddo she was coming. This made my kiddo extremely happy, and my ex felt like the star of the evening (which helped set a more positive tone for the rest of the trip).
For you guys, she may need to stay a bit farther away...maybe even at a different resort...but I dont know how well the situation is between you all. Worst case though would be you guys meet up and hand off the kids someplace neutral, and the kids can even crash with her during her portion of the stay if they chose.
After my ex got remarried, I still invite her and her husband (and the kiddos step sister) on every trip or portion of the trip. I dont offer to PAY for their portion of the trip, but they have always had the choice to join us. They have just chosen not to (though there os some rumbling about trying to do a family cruise next year)...
If you dont invite her along, or she decides not to come, I would highly recommend that you establish some sort of "share with Mom" behaviours. Meaning, set aside some money and have the 6-8 year old go shopping with their Dad to buy something for mom. Have them (and just them) call her regularly with updates...facetime is even better. And, for rides like SSE where you can make electronic "post cards", have the kids do one on their own and email them to her. This way she feels loke her kids are missing her and thinking about her, even while enjoying the trip with their Dad...and hopefully could help quench some of the fire.
Tough situation. I wouldnt penalize yourselves or the 5 year old over it...just try and either get it added by the court or take out trip insurance. Do not let mom (and it is mom) dictate your families lifestyle. If she chooses to not play ball, let her be the one to make that decision so that you and their father have a clear concience.
One final note...and this is partially why you need to go through the courts...an extra CYA...taking a child across state lines is generally a provision written into the parenting plan. You will want to check the plan to see what all is involved in that. Again, if court approved and amended, no concern. But, if his normal visitations end on Sunday evenings, and you guys are in Florida and dont return the children, the Mom could pull some rather nasty business. Dunno of she would, but a few phone calls as a hysterical mother and a father in breach of the parenting plan can lead to a mess that you dont want to deal with.
If you dont go the court route, at minimum get a written and notorized agreement (not an email or text message) and the trip insurance. But, I think you should go through the court (my highest recommendation).