Is it permissible for children to prevent their father from selling a large portion of an agricultural land they own, in order to buy a big house for his other wife and her children, given that he has many debts and their financial situation does not allow for it, and knowing that he does not respond to their advice?

Preferring some children over others in gifts and endowments is not permissible. The father must be just among his children in expenditure and gifting, because this is not equitable. If a father violates this and prefers some of his children over others, he incurs sin, and his reckoning is with his Lord. It is not permissible for children to prevent their father from disposing of his private wealth, because this is considered an interdiction, and interdiction is only carried out by a judge's ruling against an incompetent person.