I'm not trying to say that there's no difference between being a parent and not. Of course there is, a massive one. The question is whether it ultimately makes people happier or more satisfied and how anyone can know if it would / would have.
Those with kids of course have their lives tied to the ups and downs of their children and a large quantity of their time and energy becomes devoted to this. But for those who don't have children, the equivalent time & energy doesn't just go into a vacuum instead. It goes into other things, other human relationships (that might include other people's kids), projects, and causes and these may prove more or less fulfilling than kids would have been - no-one can ever know for sure.
Lurking behind the question, I think for many, is the suspicion that kids is the "natural" thing, the "natural" way to be fulfilled but this might rest on a number of inaccurate assumptions. For example, being a parent now is almost certainly a very different thing, with very different expectations, from being a parent 500 or 4000 years ago.
I don't find 'happiness' a very fruitful concept tbh. Hence why I didn't use it.