I'm not sure it's regret having children per se. I think for many people it's the amount of children they have.
One is joyful, or is and was for me. But I think when you start to get to two, three, four, or more, it becomes very very different. I know with my friends with more than one it's very different, from costs, to childcare, to sibling rivalry to differing needs, to additional work.
And I see it all the time on here, about not having an only child. I always think why the hell not. So many siblings hate each other, it's stressful for the parents, expensive, and takes over your life the more you have. With one you're more disposable income, more time, and you can still have a life. And it's a sliding scale down. And the more you have, the less money, time and life outside kids you have.
Wanting a child is a natural instinct for many people. I think understanding the implications of having many children is what potentially leads to difficulty and regret.