To clarify, I didn't say it would be rude not to invite Casey; I just thought she might feel hurt to be the one person excluded, especially since she wouldn't be able to share the experience with her children. Nothing can be done about this since there isn't room for an extra adult.
It's too bad that your wonderful generous offer is getting so bogged down with etiquette issues.
I do hope the discussion doesn't dissuade the OP from making the offer at all, as I think it's an inherently generous gesture. I just think it's good to think about other perspectives. I certainly don't think the OP is trying to be controlling or anything, but you never know what someone else has experienced and/or might be sensitive to/worried about. And, group dynamics are complex.
think it's rude to invite Casey's kids but not her. But, I think it's important to realize that Casey may not automatically agree with the suggestion, or find it positive, for a wide variety of reasons that are no reflection on the OP. I think the OP should decide what she wants to and can afford to offer, and whether there's any room for negotiation in that, and then make the offer, and if Casey declines, take that in a good spirit, which I'm sure the OP would anyway.
For the practical details, I can see how the meal plan's all-or-no-one setup makes it tricky. The OP and her DH know they prefer the meal plan, but to have it they must "force" everyone else to have it too, either the OP paying it for everyone, or everyone paying it for themselves.
I think the OP and her DH fundamentally have to decide what their hard limits are, and where they can be flexible. I mean, they wouldn't pay for people to stay somewhere else
, right? It's going to be the DVC they want. So that's a hard limit. If someone objects to the meal plan, what are they comfortable doing--not having the meal plan at all, paying for it for the guest (if money was the concern), regretfully withdrawing the trip offer from that guest? Are they definitely expecting that the whole group will be in actual WDW all or most days, or would they be cool if some guests went to the water parks instead, or lounged around the condo all day? Would the OP and her DH happily join the guests away from WDW or would they go their separate ways? What's the hard limit on what the OP and her DH are able to spend on this trip?
Then I think I would talk to John and Samantha first, not just about the definite things but also about what you hope for from the trip. "We'll pay for the condo and the meal plan, you guys get yourselves there, and our expectation is you'll buy your own WDW park tickets so we can go there together every day." And see if they feel the same way, or if they have different expectations that you can be flexible about. Once you get things worked out with them, then maybe you can approach Casey with definite numbers--she'll be paying for her own kids, but they'll be under J&S's supervision and will presumably go with whatever J&S have agreed. "We're paying for X services, you'll have to pay $Y for park tickets, arrange the kids' transportation down there, and give them money for incidentals, $Z being a common amount for that." And do have some flexibility, in case the older kids have some ideas about things they'd like to do, maybe rides that the rest of you wouldn't be interested in, or something like that. (And, if Casey declines, I wouldn't change the budget from what J&S have already agreed to.)
I think problems happen when one person goes into this assuming that everything is going to happen a certain way, the way they
enjoy, and then they find out that other people have different assumptions or expectations, maybe so different that one side or the other is no longer having a good time. Just basic vacation compatibility stuff--one person wants to get up at 6am and go-go-go to see everything possible (which the other one finds exhausting), and the other person wants to sleep late and lounge around more (which the first one finds wasteful). And if one person is paying for the bulk of the vacation, it can feel like they're using that leverage to "make" the other person do stuff--which to some extent is certainly reasonable.
So I think it's really important to communicate the expectations clearly with everyone, know where you can have some flexibility, and also know your limits, so you don't feel pressured into agreeing to something you really don't want/can't afford to do. That's all, totally easy, right?