A few points:
- The term is people of the book (ahl-ul-kitab), not “children of the book”
- The term “people of the book” refers to non-Muslims who belong to monotheistic faiths and whose sacred texts are recognized by Muslims. The term generally only refers to Jews, Christians, Sabians, Zoroastrians, and never refers to Muslims.
- People of the book are not considered to be “believers” - that term generally only refers to Muslims.
- The Quran specifically makes an exception and says that Muslim men can marry women from the people of the book. But that is not the issue being discussed here.
- There is no such exception for Muslim women, who are categorically forbidden from marrying outside the religion.