It seems like if you want kids and are very concerned about climate change you can do both. Just purchase carbon offsets.
By my own calculations, it looks like offsetting 100% of an American's carbon footprint costs $55/month.
Sources:
http://www.treehugger.com/renewable-energy/survey-of-carbon-...
http://www.ecosystemmarketplace.com/articles/carbon-offset-p...
9,441 metric tons of carbon * $5.5 /metric ton / 936 month average lifespan = $55 / month