Maybe they are unhappy but on the flip side, most people with children will tell you that if you haven't been a parent you don't know what happiness is. The happiness of being a parent is just unimaginable, cannot compare with anything else.
I mean, what is parenthood if not love and voluntary hardship?
On the other hand, I think you are describing your subjective experience. I've talked with some "one-and-done" parents who deeply love their child, but wouldn't want another one if you paid them.
"Happiness" is one of those words that has thousands of different definitions, so I would frame it this way: Parents almost always describe their children as the greatest joy in their lives, but (in America at least) they also generally express more dissatisfaction and frustration with their lives than childless people.