What is Islam's stance on a Muslim woman raising her Muslim husband's children?

Your action is an act of benevolence, and for it, you will receive the greatest reward, especially since it is benevolence towards the weak, as stated in the Hadith: "In every wet liver there is a reward."

We advise you to raise them with an Islamic upbringing, such as teaching them Tawhid (monotheism), and the meaning of "La ilaha illa Allah, Muhammad Rasul Allah" (There is no god but Allah, Muhammad is the Messenger of Allah), and warning them against what contradicts that. Also, inform them about the Last Day, Paradise, Hellfire, the fear of Allah, and His love. Correct misconceptions regarding issues that have been distorted by the enemies of Islam, and present the missing concepts that a Muslim needs, so that they may grow up with a strong foundation of faith.