Refer to science. Start here:
https://thepsychologist.bps.org.uk/volume-22/edition-4/think-having-children-will-make-you-happy
A chronicle outlining why having kids makes you unhappy.
And here are some research on the subject if people don't believe this article (note that it is written by a professor in psychology and published in The British Psychological Society's magazine)
http://www.pnas.org/content/107/38/16489.full.pdf
http://link.springer.com/article/10.1007%2Fs13524-015-0413-2
http://users.wfu.edu/simonr/pdfs/Simon%20Contexts%202008.pdf
http://personal.lse.ac.uk/mackerro/happy_natural_envs.pdf
Basically, what all these papers say is that having friends, a partner, interesting work, pets, good friends, stable economy and housing, hobbies, being in the countryside, city centre (but not suburbs = where you move when you have kids), mountains, near the ocean or a lake and so on - everything is very intuitive - makes people happy. But spending times with your kids has the opposite effect and therefore, a lot of people planning to have two kids stop after the first.
If science reports are too impenetrable for the people arguing with you, here are some other articles in regular news outlets and similar containing the same information
https://parenting.blogs.nytimes.com/2009/04/01/why-does-anyone-have-children/
http://www.psmag.com/health/do-children-make-us-happy-60392
Food for thought: most western societies heavily subsidise having kids. In northern Europe just one year of school probably is around €15 000/year (or more). Add healthcare, sport and social activities to that. And still, in most countries the native population is decreasing. My conclusion from this is that people, when they have a choice, don't want to have kids. Imagine if car ownership was subsidised by €15 000/year - everyone would have one or more car(s). And imagine if children weren't subsidised but instead parents were obliged to pay for schooling etc from their own wallets and really could feel how expensive it was: "Hmm, should I spend €15 000 on education or should I take a two months luxury vacation in the West Indies and still have money left. That is a difficult one to answer…".
