I don't know if I qualify since we are on path to FIRE, but haven't reached that milestone yet (will likely be a 4-5 years before we hit FI, and will likely keep working on more retracted basis afterwards).
So far parenthood has had a fairly negligible effect on our FIRE path, while having a much bigger impact on our social lives than expected. I attribute this to two things;
i) kids don't HAVE to cost a ton of money
ii) we already had a sizable 'stache of investments, an E-fund and health insurance before we had our first child.
Despite all the marketing and scary articles about kids costing '$210,000 each before college!' this doesn't have to be the case. Between family gifts (some), hand-me-downs from friends (a ton) and a few consignment purchases we spend maybe $50/month on our baby (outside saving for college and health insurance, covered below). There's such an enormous surplus of kid stuff in this world, and kids grow out of it so fast. We also started contributing $100/week to a 529 college fund before she was born, which will be more than enough by the time she's college aged - in fact my calculations show that we'll be able to stop contributing entirely before she enters elementary school and compounding will do the rest.
Having employer-sponsored health insurance has been key in keeping all costs down; interestingly it cost less to add DD to our plan than it did my spouse, but at an additional ~$28/mo we've recouped over $7k in medical expenses the first year alone due to some medical issues and lots and lots of doctors visits. We anticipate similar bills in year 2 but again, this is covered by insurance so the net cost to us will be under $400/year.
Daycare could be a considerably cost (~$1,100/mo in our area) but so far we've been able to avoid it entirely. My spouse works at home (which has proven immensely challenging with a young child) but I have a flexible schedule where I can also work from home 2 to 3 days a week. Our strategy has been to basically alternate which parent is working and which is watching the child and/or doing housework. THe downside is that this means one of us starts at 6am and the other stops around 9pm, and at least one of us has to work at least one day over the weekend, but it's our approach to not putting her into daycare just yet. When she gets older we'll do limited daycare, but that will have as much to do with social interaction as it does with freeing us up to do other things (both will be important!). Personally, we wanted specifically NOT to put her in daycare for the first 12 months for a variety of reasons (including breastfeeding, disease, personal interaction, cost). But it's a decision each parent(s) will have to make on their own.
Finally - having a strong social network has made all of this possible. We've been unable to have much support from our families (too far away), but our workplaces have parent-friendly policies, a number of our friends have recently had children (hand-me-downs!, advice!, emotional support!), and our community has a lot of groups for new parents. Without them I don't know how we would ahve survived without spending a crap-ton more money. If you are expecting, or even considering becoming a parent I'd put some effort into shoring up these networks now. That will pay you dividends, while reading those articles about baby-development and potential childhood diseases will be relatively useless and scary.
hope that gives you some good input...
~n~
...also, hopefully
@arebelspy and
@sol will chime in, since they FIREd with children while still in their 30s and 40s.