So your expectation could be clickbately phrased as "couples with kids are less happy"? Why did you expect that?
Edit: link: https://contemporaryfamilies.org/brief-parenting-happiness/
Not all research findings fit with each other.
> particular how happy various demographics are with the part of their free time when they can do whatever they like
It wouldn't seem all that surprising if parents wished they'd have more of that, and is something totally different than "couples with kids are less happy".