First paragraph:

    Parents often say their children bring them joy
    but most research suggests kids cause financial
    worry, stress and anxiety.

Second paragraph:

    However, the latest research suggests children
    do actually make people happier – but only once
    they’ve left home.

That is in line with what most studies come up with as one of the central aspects of a happy life: Having friends. I would think that kids kinda fall in that category after they left home.