This thread has been a very interesting read! We are pondering these same questions as we try to figure out what our expenses will be as they get older.
Re: Vehicles
When I first got my license in high school, my parents provided me with an old beater vehicle to drive (hand-me-down), but I had to pay for gas. They covered insurance until I graduated high school. After that it was on me, so I opted not to take the vehicle to college.
I'm not sure what we'll end up doing. Our current vehicles are old enough that they do not have a lot of the safety features that I would like our kids to have in their vehicles. So we may opt to get vehicle instead of just giving them our current cars. I think at a minimum I want them to have a backup camera. My oldest is 12, so we've got a bit of time.
Re: How much financial support after they become adults
Right now we're leaning towards the approach my parents took-- They told me they would provide 50k (in 2002 money). They explained that their hope was it would be used for college and living expenses, but how far it went would be totally up to me and anything remaining was mine to keep. I'm a maximizer by nature, so this was very incentivizing. I began attending college while in high school (PSEO), allowing me to get two years of college for free. Then, I opted to go to an in-state school where I received hefty scholarships. I worked throughout college and took as many credits per semester as I could (because the first 12 credits were the most expensive each semester, and any beyond that were discounted). In the end, I graduated with my Master's Degree at age 21 and had only $15,000 in loans.
That 50k was it. They didn't pay any of my bills, buy any clothes, take me on vacations, or what have you. I was fully financially on my own the moment I had access to the 50k.
I really appreciated this approach in part because of how clear-cut the line of financial separation was. There was no ambiguity over who would be paying for what anymore. This is in contrast to DH's parents who never had the conversation with their kids, and as a result still end up footing the bills for his sister (who is mid thirties and earns more than both of them combined). They grumble to us about this all the time, but of course the one they need to be talking to is the sister.
Re: Have you accounted for this in your FIRE budget
This is exactly where I am right now. I'm trying to figure out what/how/how much support we'll provide over the years. I like the idea of doing a clear cut total, similar to what my parents did, since that seems to be the easiest to budget.
Re: Do you feel like your parents should have given you more financial support
No, with a caveat. I think they way my parents managed those early years of adulthood really worked out well. It encouraged me to be mindful of spending and aware of the true cost of each of my choices.
Fast forward to the present, and I am nearing 40 and they are mid to upper 70s. My parents were solidly middle class earners, my mom working in education and my dad held a blue collar job. They were frugal and intentional, managing to retire at 57, and have now accumulated quite a sizeable nest egg that continues to grow each year. At their request, my sibling and I have attended their financial estate planning meetings so we know just how dandy they're doing. They could double their annual spending and still have a growing portfolio.
With this in mind, there have been some times I have wished they would do an early inheritance. The money would be far more helpful to us now than it will be in (hopefully) 20 years when they pass. By that point, I sure as heck hope we're already FI. Sure, it'll be nice to know there is a cushion for us in the future, but it sure feels like it would be more useful to us right now.
We'll see how things are going when the time comes, but right now I am envisioning meting out my inheritance to my children once I receive it. I hope to be in a position to no longer need it for myself. But I don't plan for or say anything of this to my kids, as it'll totally depend on if I feel like they're making smart choices or not at the time. I'm not interested in helping them to buy a bigger house than they need or a fancy boat.
