Perhaps it is strange that my first thought was NOT how much it takes to birth, house, feed, clothe, school, etc. etc. kids from birth to 18 (or 22?). Rather I thought of kid's impact on SOME parents after kids would (otherwise) supposedly be GONE. IOW the effect of adult(?) kids when mom and dad DO actually reach the time to retire.
I know so many couples who have either NOT retired because of the needs of their adult children (including "needs" of the grand kids) or have retired and then been virtually bankrupted by the "needs" of their kids.
Just among our close friends: One couple sent their adult son to 1) welding school 2) two different undergraduate programs 3) graduate program 4) certification training for physical therapy. The kid was married with two children and in his early 30s before he EVER took a job beyond mowing lawns, etc. The couple eventually retired as virtual paupers. Don't know what happened to the kid as we have lost track of our friends. We could no longer spend evenings with them, listening to their continuing tales of woe. They never listened to our advice but they did ask us for money (a good way to kill a friendship.)
A current friend who w*rked with me at Megacorp retired very early (age 50) with a "package" which I was not eligible for at the time. That was 22 years ago. He is still bailing out (literally) his dope smoking son. He supplies son with cars so he can get to w*rk to meet the requirements of his release. Son recently got a DUI and had a firearm in the vehicle - and then resisted arrest. To save his son from a year in jail, friend paid for ankle bracelet/monitoring system and AGAIN got him a car so he could find a new job. Friend can barely feed himself and wife and keep his own car and house going. Friend's wife spends hundreds of dollars per year purchasing "gifts" for their daughter who is a professional with good salary. These folks have ruined their retirement because of the perceived "responsibility" toward their kids.
Another couple just retired at 62. At the end of the month, they eat whatever is left in the pantry (tuna helper, mac and cheese, canned foods, etc.). Their two daughters (about 30 and 35) consume most of the "spare" money and almost all of their time. The 35 yo is a professional, but "expects" mom and dad to drive 60 miles, one way, to baby sit so she and hubby can go out. 30 year old, still recovering from a destructive divorce expects mom and dad to keep her horse boarded and supply her with a farm truck and a car so that she can work her medial job but still ride her horse. The couple HAD to purchase a 5th wheel camper (and a big truck) so that the extended family could take vacations together or just go camp at the lake most week ends.
I could go on, but, full disclosure, we have provided each of our kids with either a house down-payment gift or business-starting gift in the range of $10K to $20K. The difference is that the gifts were in our plan and they do not materially affect our retirement. It's true that we don't (yet) fly first class and our gifts certainly could have made that possible. But, in truth, we could still fly first class but choose not to. Additionally, we give each kid at least $1k per year toward their Roth IRAs. ALSO in the FIRE plan.
Now returning you to our regularly scheduled discussion of those who have delayed retirement due to the costs of actually raising kids from birth to "adulthood." YMMV