I think the people who regret having children tend to be the ones who give up the most freedom.
I ofte see this happening to friends and family members, who spend all of their waking moments catering to their children - the ones who have given up good careers to be SAHMs for a variety of reasons, the ones who try to combine work with their children's intensive after-school schedules (I know one mum who spends 6/7 days ferrying her children around to various activities after work), those who have uninvolved partners and those who feel the pressure to keep up with the social media image of having children.
Childcare nowadays is far more intensive than it was 100 years ago, where children would be sent to play out for hours on end and often, even at a young age (say, 6 years and up), disappear for a few hours to play with friends. Over-protective laws and the idea that children need 24/7 entertainment have a lot to answer for.
Example, my mum did not need to worry about before- and after-school care as I was walking myself to and from school aged 6 onwards, and as a typical latchkey child would reheat food once home and then just entertain myself for a few hours until she came home. Unthinkable these days, SS would be involved in no time. My children's primaries both explicitly stipulate that children must be brought in and collected by an adult until year 5 - aged 10. That alone adds and extra 4 years' worth of pressure and monetary issues.
That's not even to mention the idea that a typical childhood weekend for me would have been "stayed at home and played", while when you read the school teddy's adventures now, which display a "typical weekend" appear to often involve theme park trips, intensive games, various out-of-school activities and so much active involvement, children whose weekends were like my own childhood ones would be seen as having a boring life.
TLDR, pressures have shifted massively, making mums' lives fade in the background in preference of a fun-filled childhood for their children. No wonder so many women are now miserable.