What is the ruling of Islamic law regarding a husband who promised his wife before marriage to quit smoking, then later returned to it, and is the wife entitled to seek divorce if this harms the upbringing of her children?

Your husband is not permitted to smoke because it is forbidden (haram) and harmful to him and those around him. It is not permissible to seek a divorce for this reason. Rather, you should offer sincere advice and explain the harms of smoking, seeking help from books, audio recordings, and fatwas that discuss this issue, while also monitoring the children's behavior and warning them against anything harmful.