Let me share my thoughts while I am keeping awake since my toddle is sick.
Cons:
1. Time. Consider spending 3-4 hours on weekdays, and almost the whole 2 days in the weekend with kids until they turn 6 or 7 (i.e. the age they can start taking care of themselves). This is during normal time. When a kid is sick or the nanny does not show up or the day care is close, you will have to spend more time.
If you count these are about 40-45 hours a week. I don't know about others, but I would probably be spending about 30 of these hours on research if I had this extra time. I feel my research time has become less than half of what it would be otherwise (I am at an R1).
2. Your wellbeing. Less sleep, intermittent sleep, extra cooking, crying, feeding when they are not cooperating, or even simple tasks like going to grocery can become very difficult. I expect it to ease once they can take care of their own.
3. kids require money. We spend about 40k on child care per year. This is mostly post-tax. There are additional costs too. You do the maths.
4. kids affect your attention. I don't feel I have the same type of attention for work as I had before. This may be just me though.
5. kids may affect your emotional well being if they are not very healthy or not very cooperative. This is basically a lottery.
6. Kids will crowd out your social space. No new friends, not meeting old friends, less travel and even when we travel, we cant meet friends in the same way.
Pros.
1. Kids are fun. You look forward to going to your home in the evening. It's a great feeling to hug your own.
2. Kids give you perspective. Like you understand life better than before.
3. Kids become easier with time. But that time does not come till they are 6 or 8. They are out of the nest by 18.
There may be many more pros.
The extent above factors may affect one's life will vary with individual's constrains (money, extended family, partner job etc.). My view is that one should have a kid of the trade off does not look too bad. However, having kids too early may not be as good as it sounds. This may affect your career path altogether. The same applies for having kid when you are on a tenure track. That said, you should not really time it so strategically.
This is surprisingly good. One kid is manageable. We might have a second, but who knows. We have a 4 year old. We still travel, but less. We still go out, but less often. As far as work...people on this board seem to miss this fact: WE AREN’T CURING CANCER. QJEs and AERs aren’t that important in the grand scheme of things. Heck, even our Nobel Prize is a fake one. It’s a lifetime achievement award, not an award for best discovery from the previous year like the ones Alfred Nobel funded. Having a small human that loves you is pretty damn cool. And I wasn’t the biggest fan of having kids...before I had one. But, watching some small human have to learn all these basic things we take for granted is fun. Plus, the unconditional love is quite cool. You kid doesn’t care if you get an R&R. They care if you get excited that they figured out how a doorknob works or spell their name correctly.