Probably unfair by person, but fair by room? I think it would still be cheaper for you than your old 1/4 share so they’d actually be subsiding you!
Why would readjusting the split to make it fairer for everybody mean that OP and DH would then be being subsidised? 
I don't get the 'justification' that children don't earn anything and so shouldn't be paid for at all. Of course the children won't be paying for themselves, but their parents should be covering their own children's costs, if they're having their own rooms and thus increasing the overall cost for a larger villa.
Children cost money - everybody knows that; and that's why most responsible people consider those costs before they decide when/whether to have children and how many. It is a CF move to assume that other people will pay for/subsidise your children.
Nobody would be suggesting that a family needing a 3 or 4-bedroom house should expect to pay the same to buy/rent it as the childfree couple in the 1-bedroom place next door pay, just because the children themselves don't earn any money (except they actually sort of do, if you include child benefit and any child tax credits, which a person/couple without children will obviously not receive).
As for buying rounds, couples expecting to be counted as one 'unit' when drinking with a group including single adults should be smartly disabused of that notion. Fair enough if they're one of the many couples where the woman never goes up to the bar herself, but assuming they're both drinking, that means the man needs to go twice as often - once for his own turn and once for hers.
Having said that, rounds only really work if everybody is drinking pretty much the same thing - they're ripe for the CFing if you have people seeking to take full advantage of being subbed (ordering a Coke when they're paying and a double whisky when somebody else is).