If you're eating out, I think paying per head is the right thing to do. If you're buying the ingredients for the evenings dinner to cook at home, does it really matter if you pay £2 more?
Generally speaking it's much easier to find a system which doesn't involve le doing a division of some type for every individual spend ....instead, someone pays for one expenditure and someone else for another and you keep track of your spending and work it out T the end of the week, based on a fair system, which T that point could easily recognise you and less adults. This sounds much better than trying to pay a share for each piddling ice cream spend or trip to the supermarket, especially if there are several.
And if you really are only prepared to pay to the penny for what your own family have had in all areas, group holidaying really isn't for you....you have to be a little bit flexible about all kinds of things on a group holiday - bed times, activities, etc etc or it just doesn't work. So, if overall there was no recognition of you being a smaller family, I think you could feel aggrieved, but over every I'd civduL little expenditure, I don't think it's needed to be honest.
As always, clear communication about how these things will work, upfront helps a lot. And again, avoiding frequent splitting and handing over of money, which isn't always easy is best.....one big reckoning at the end, or a kitty system at the start are best.