Um... Don't quite know how to respond to this.
I'm not the single person in this group, and I don't know what I have posted that would give you that idea. So the majority of your post was labour under a delusion. In actuality, I am one of the people in the childless couples, specifically the Gail and Harry of the example. I posted this to EHell because someone in our group had made a proposal that I thought was slightly unfair, and it got me thinking about what the fairest split would be.
I have been nothing but civil in my responses to people on this thread, and I'm not quite sure why anything I've posted warranted this quite aggressive diatribe. This is a discussion forum, and my suggesting a particular course of action is not an attack on anybody else's belief system.
If you found my "diabtribe" aggressive, then I do apologize. I was not trying to be aggressive. I also apologize for assuming that you were single.
The bottom line for me, is trying to calculate 'common space' into the cost is too complicated and doesn't really solve the "fairness" question. I don't think that common space is 50% in this case, and even if it was, I don't think there's a way to make the calculation that works. I don't think that couples get a benefit out of paying the same amount for one bedroom as a single person would for the same bedroom.
If the breakout of costs that your friend proposed doesn't work for you, then I do think you should say something. But, I honestly believe your trip would be much more enjoyable if your group came up with the simplest method possible to break out the costs. If they agree to something more complicated, that's great. But, for me, I can't see coming up with a more creative calculation as anything more than a waste of time. It wouldn't be worth the money I'd save if I was a single person and honestly, makes the it look like the single people are just looking for a reason to be put-out (even if this isn't coming from them).
ETA: If you are Gail and Harry in this scenario and you would like to offer to pay $333 for your room while Isobel pays $167, then I don't see why you can't offer that to her. The $278/person breakdown, I agree, is not the best way to calculate it. Because it puts those that are only staying half the time at a complete disadvantage. But a per room/per night breakdown is still the simplest while also being reasonably 'fair' and it's the one that I would still suggest. You can always offer to take on more of the burden from Isobel since you will be splitting your room with her if you feel strongly about her not getting a fair deal in this. I wouldn't worry too much about the extra costs of the kids. At such a young age, I wouldn't expect them to even eat half or a third of what the adults eat. They will be sharing rooms with their parents, and parents will likely be bringing specific snacks and drinks for them anyway (with them only taking a small portion of the communal food during meal times).