Are children sinful if they arrange their father's marriage despite their certainty that it will harm his wife?

It is advisable to seek a righteous wife for your father, as this may improve his condition. The woman being proposed to must be informed of your father's situation, in accordance with the Sunnah that called for offering advice to every Muslim and the obligation to disclose the shortcomings of a suitor, even if advice is not explicitly sought. This is confirmed by what Al-Hakim narrated, that Bilal disclosed his brother's shortcomings to a woman who intended to marry him. Therefore, whoever does not disclose their situation has sinned. You should supplicate for your father's rectification and for someone to be empowered to advise him.