I think this is an important discussion to have because many vegans, for reasons of their environmentalism, seem to be against having children. The fact is, if you don't have children, other people will. And their children will most likely not be vegans. And that means more animal suffering, more environmental devastation, and probably negative financial consequences for society (in the form of higher health care costs).
Alternatively, you have children. Sure, that might create some problems for the world... but ultimately, it's one way to bring more vegans into the world. And that has a range of different benefits:
1) More vegan voters ---> increased political strength of vegan movement ---> legislation to promote & spread veganism, subsidize meat alternatives, tax meat, etc
2) More vegan food consumers ---> increased consumer spending on vegan foods ---> increased corporate interest in producing vegan products that make it easier for non-vegans to go vegan
3) More vegans living in society ---> increased public exposure to vegan ideas through normal social interactions ---> more people decide to go vegan as they learn about it
4) More vegans in society ----> media companies produce media content to appeal to a vegan audience, which spreads veganism more rapidly through society
One argument against having children is that the children may decide not to be vegans. However, I think this is unlikely. Studies show children tend to have similar political views and dietary habits as their parents do... and veganism is obviously political and a dietary habit.
And if you are a committed vegan, you will have many opportunities while raising your kids to make sure they understand why veganism is important. You can have them watch a range of different documentaries and TV shows that provide a perspective that supports veganism like Cowspiracy, Dominion, Forks Over Knives, Speciesism: The Movie, Lucent, PlantPure Nation, ...etc. You can have them read and study books like The China Study, and look at the scientific research on websites like nutritionfacts.org explaining why animal products are so unhealthy.
Having children is a tremendous opportunity to change the world, and no vegan committed to changing the system should fail to seize that opportunity. The more the merrier.