If you're saying that the kinds of people who typically wish to be poly are the kinds of people who typically don't want children, that might be so, though I haven't seen any evidence for that hypothesis. Anecdotally, the "wants children" / "doesn't want children" ratio seems about the same as in the general population, or maybe as in the general high-IQ population. Your original comment seemed to talk about the suitability of poly for raising children, given that the people involved want children, though.

But I actually think that the main benefit of having three parents is for the adults, not the kids. Child-raising is typically really, really tiring, at least when the children are still young enough to need constant supervision. Having a third person around would really help make things easier. At the same time, there are all kinds of studies around saying that most of the things we'd expect to have an impact on the long-term outcomes of the children actually don't, and I'd guess that this would fall into the same category.