Taxes (a little wonkish)
I think that this tax conversation is conflating a couple of different things. Individual nations use tax policy to further that nation's economic priorities, which are entirely seperate from any concern for the species as a whole. Furthermore, this "states as rational actors" summary doesn't even give the whole picture, because many states wind up with counter-productive policy through path dependence, mismanagement, regulatory capture, etc.
To bring this back 'round to being poly- and child-free, those are two groups that are very badly served by tax policy in those countries that I know enough about tax policy to comment on. While not being in any way part of the child-free movement, I absolutely support any effort by either group to advocate for tax policy that better serves them. I'm tempted to go even farther and say that there is a way in which they should be allies in that advocacy, because both groups are opposing the currently privileged nuclear family paradigm, but I think such an alliance is pretty unlikely.