Is it permissible for a husband to hit his wife who hits their sixteen-month-old son?

The child in question is young and not discerning, and there is no benefit in hitting him. Punishing him with physical beating at this early age is an unlawful torment. Advise your wife not to hit him. If she does not comply, threaten her with hitting and punishment. If that does not work, then you may hit her in the usual manner, one that does not break a bone or disfigure a limb; for a husband disciplining his wife is permissible.