Is the statement "Children are misery and an illusion in this life" considered disbelief (kufr), given that its utterer heard the verse "Wealth and children are the adornment of the worldly life" and said it without intending to deny or mock, and has since repented?

A person does not become an apostate by saying "children are a tribulation" as long as they do not intend to mock the Quran or disbelieve in it. It has been narrated in the Sunnah that children are "a cause of cowardice and sorrow," meaning they might lead their father to cowardice and sadness, and "a cause of stinginess, ignorance, and sorrow," meaning they are a reason for stinginess with money, abandoning the pursuit of knowledge, and excessive sorrow over their illness or demands. Wealth and children are indeed blessings from Allah, but true happiness with them is achieved by utilizing them in obedience to Allah and raising children [in the right way]; otherwise, they become a source of misery.