There's an interesting article in the New Yorker exploring whether it's wrong to have kids.
The author's main concern seems to be overpopulation, but would-be parents can easily offset their impact on global population size by making an appropriate donation to Population Services International or the like. If the result of their choices is that more children are born into prosperity, and correspondingly fewer born into poverty, then that can only be a good thing. And even without such an 'offset', I'd expect the marginal child born to upper-middle class Americans to produce more positive than negative externalities over their lifetime. (As Caplan says, "More people mean more ideas, the fuel of progress.")
A better argument against procreating may be the lost opportunity for adopting/"saving" children from orphanages, etc., where they would otherwise lack the sort of care and opportunities that you could offer them. Adopting children in need is certainly exemplary, though presumably supererogatory if anything is. Setting this ideal option aside, then, I think we can at least say that for many people it is better for the world that they have and raise biological children rather than doing no parenting at all.