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.

Strangely enough, I think I do understand. As near as I can tell, life's two greatest pleasures are 1. Love (both loving and being loved) 2. Voluntary hardship

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.