Some philosophers argue that we should limit procreation – for instance, to one child per person or one child per couple – in order to reduce our aggregate carbon footprint. I provide additional support to the claim that population size is a matter of justice, by explaining that we have a duty of justice towards the current generation of children to pass on to them a sustainable population. But instead of, or, more likely, alongside with, having fewer children in in (...) each family, we could also create families with more than two parents. I explore this possibility by pointing out the ways in which multi-parenting can advance children’s interests: in higher levels of well-being, in non-monopolistic child-rearing, and in a future opportunity to become themselves parents. (shrink)