>> but when these people are asked whether having kids made them happier they would say yes

This can be quite easily explained by peer pressure - You are expected to treat children as precious little gifts from god, and speaking ill about them makes you look bad in the eyes of your peers.

Anecdotally I've asked close friends for genuine answers and from my perspective they seem convinced they are happier.

That being said I have also (fewer) close friends who have told me they are less happy. So you have a point.

They are in a situation they now cannot realistically change and that they are 'supposed' to enjoy. There is a lot of pressure to convince themselves they are now happier 'overall', even if actually each of their days is now less happy.

The fact is, you will encounter plenty of misery in life regardless. You should not choose based on what makes you happy, but arguably based on what kinds of misery you are more suited to - the misery of having children or the misery of not having children.

I'm sure that there are people that genuinly create happines from interacting with and having children. Just like there are people that takes pleasure in hurting themselves. The real tragedy is You cannot know which one are You until is too late.