Yet, there are a lot of those things you can still do with kids around without neglect them. That's a part (in my opinion) of education to involve them in your activities.
Of course, it's not true on their first years, but when they start to grow, that's something you can do.
For example, last December I went with my wife and my 7yo twins to a 10 days vacation in NYC.
They enjoyed visiting the Met or the natural history museum, probably not in the same way my wife and I did, but still. And it was a great time for all of us.
And kids go to sleepl early (for their well being), ours are in bed at 7:30pm, so there's still a lot of time to do plenty of stuff :-)
This attributes way more intentionality onto having children than actually is the case, in almost half of cases.
In 2011, 45% of all pregnancies in the US were unintended, and 18% of all pregnancies being unwanted [1]. That means that for the nearly 4 Million births in the US yearly (using 2011 numbers), the intentions you state about how to think about time management with the additional kid likely does not apply for about 1.8 Million of them.
Speaking as a father of three, your statement is discounting a number of things - including personality type - and I find that those with kids who do a lot are predisposed to doing so, how they manage that with kids varies by their predispositions.
So its naiive to equivocate having kids to taking on a new hobby or a job - for various social, legal and ethical reasons. They just aren't the same.
[1] https://www.guttmacher.org/fact-sheet/unintended-pregnancy-u...
You may say that it is unthinkable. But it's just you choosing one cost instead of another. Being a decent human or not is a choice, being a single paying alemony or a couple hating your kids is another choice. And there is a price to all of them. That's what a choice is.
A choice is not about doing whatever you want. That's omnipotence. A choice is chosing what to do with the piece of block the life tetris gave you.
But of course, none of this argument matters because I'm answering to HN readers, a niche a very educated people that know all about pregnancy, contraception, abortion and that having children has a huge impact. This post is, after all, about somebody interested in the process enough to trigger a whole debate it.
So my comment is to be taken in that context.
Reading, cooking, handing out with friends, traveling, sleeping, working on personal projects, running, meditating, trying a new life habit, taking a break, organizing, fixing, improving, hiking, rock climbing, creating a new company, catching up on my field of expertise, cleaning, calling people I like, going to the cinema, going to a show, watching tv shows, testing a different tool, meeting new people, doing something that takes time but saves money, going to an event or a party, going to a museum, doing live escape games, playing laser tags, diner with friends, varying on sex, going to a health specialist, caring for my skin or hairs, choosing second hand cool clothes in a thrift shop, working more hours, helping friends with something...
Kids has nothing to do with what I do though. It's a matter of time. Some people without kids still don't have time. They work a lot, or have a hobby that fill up the day or just do things very slowly to enjoy the moment, or live in area where everything takes more time, or have a big commute.
Some people do have kids but have a lot of money and don't wish to spend time with them, hiring somebody to do it. Other just neglect the kids.
So the thing is more: you created a life with kids, you decided to take care of those kids, you agreed it would take a lot of time to make it the way you think it should be done (e.g: they being sane and healthy) and allocated that time to those kids.
Basically it's just like anything else: you decided to add something, here taking care of the kids, that takes time instead of something else. Kids is just a label to it and is no better, or worse, than any other activity.