We have a 2.5 year old and a 9 month old.
Major costs:
Fertility treatments - this did not go as planned. Having children was important to us, and things were far more difficult than we expected. Probably $30k between the two of them, I didn't keep track because it was too depressing.
Prenatal care and birth - about $2500 each time with good insurance.
Family health insurance premiums - my health insurance contribution went up about $400/month when we added #1 to our insurance.
Maternity leave x2 - DW got 6 weeks paid and 6 weeks unpaid with #1 and 4 weeks paid with #2. I wasn't working when we had #1, and got 4 weeks at 75% pay and 8 weeks unpaid with #2.
Bigger home - we live in a 2 br with 2 kids, one of the primary costs of having kids is needing a bigger space and eventually needing to live in a decent school district.
DAYCARE - this is the biggie. We pay $1550/month per kid (we have a huge discount for wonky reasons), but the going rate around us for infants is more like $2k/month. Varies a ton by region, whether you choose and in-home/center/nanny/au pair.
Travel - once they're 2, they need an airline ticket. We travel enough that this adds up.
College - people approach this one differently. I'm probably going to work a few extra years to pay for college, because I'd feel like a jerk retiring early and leaving my kids on their own for college.
Minor costs:
Baby gear - we bought almost everything used, and had a shower with #1. The only thing we really spent a lot on was a $350 double stroller because we walk to and from daycare and wanted a specific model. Baby stuff is used for such a short period of time, there is a TON available through secondhand sources.
Baby/kid clothes - also easy to find used. The key is not being picky.
Formula - I wasn't able to breastfeed #2, so we used formula. About $100/month, but time limited and decreases over time.
Diapers - Don't do cloth if you're not interested because you'll resent every wash cycle, but we find it fairly easy and the cost savings scale with the number of children. $600 total for two kids.
Things we have not spent money on:
A bigger car
Most paid activities/entertainment - we have gone to the aquarium a few times for $8, and we do park district swimming lessons for $30 per 8 weeks. Other than that, we've forgone fancy classes and activities in favor of the park, walks, and all the excellent free things to do in our city.
Things we are saving money on:
Eating out - we did more takeout when #1 was small, but by the time she was 18 months eating out wasn't fun anymore and we'd found our stride with easy meal prep
Entertainment - kids are very entertaining, and we go to bed around 9, so Mustachian win?
We definitely spend more than we did pre-kids, and childcare costs will still be substantial even once they begin public school (before and after care, summers). But for us, it's totally worth it because we had a hard time imagining life without kids.