1&2) I think you will probably have plenty of money if you treat it as fungible.
3) There are costs associated with having kids, but these are highly variable. Clothes can be very cheap for the first decade, and then it might be harder to find things that fit which are as cheap. Shoes are always an issue since it's harder to find used kids shoes that aren't trashed (especially sneakers; usually things like boots I can find used.) They don't eat much at first, but my middle schoolers eat as much as any adult and sometimes more. They grow and need bigger bikes, helmets, etc. There are periodic expenses associated with school even if it is public school, like field trips and poster boards/supplies for projects and teacher/coach gifts and special books they want you to buy at the last minute. Around where I live the museums aren't free, so a family membership became an expense when they were little and we visited frequently. Heck, it costs us $5 per person per game to go to their school basketball games (siblings have to pay too even though they are students at the same school.) Once they are too big to sit on your lap, every time you travel, everyone will need a ticket for trains, planes, etc.
True, some of these expenses are completely optional, some have a workaround (like volunteering to help coach or keep score at games so you don't have to pay to attend), but some you will have to pay unless your child doesn't participate or you want to let others foot the bill for your share. You obviously don't have to chip in every time someone is asking for $20 for a teacher or coach gift, but realize that people are going to ask for this periodically. Just something to keep in mind, but I don't think it eliminates RE plans by any means.
On the medical side: we have been extremely fortunate not to need much medical care. My brother has lots of medical expenses associated with his children, but his limited income (he actually earns a lot, but not enough to cover some of the needs) has led to places like Shriner's hospital picking up most or all of the tabs. One of my children needs braces, which are expensive, and my vision keeps getting worse, which is becoming expensive. I'm glad you have something set aside for this.
I would say keeping a "hobby job" or side hustle active much of the time is a good idea for most people who FIRE young with a family. That way you can reduce the cut into your stash and keep your resume filled with employment dates if catastrophe ever strikes and you really need to reenter the regular work force.