Does a husband's threatening his wife with an oath not to hit their daughter again count as an oath of divorce?

Your statement, "I will swear an oath," does not entail any consequences because it is a promise to swear, not an actual oath. It is not permissible to strike a child in a harmful way. If there is a need for striking, it should be a disciplinary, non-bruising strike. If the wife's striking of her daughter is for disciplinary purposes, there is no need to object to it. However, if it is harmful, advise her gently and kindly, without anger or words that might cause estrangement between you.