Prophet's Tribe, Ahlal Bayt (Family) & Righteous Successors
Hazrat Khadija (r.a) did have children before marriage to Prophet Muhammad ﷺ. The famous 20th century Islamic Scholar Maulana Maududi in his Commentary “Tafheem Ul Quran” quotes early Islamic Biographers on the life of Prophet Muhammad such as Tabari, Ibn Saad, Abu Jafar Muhammad bin Habib (author of Kitab al-Muhabbar) and Ibn Abd al-Barr, who mention that she had 2 husbands and gave birth to 2 children from each:
“(The Biographers) state on the strength of authentic sources that before her marriage with the Prophet (peace be upon him), Khadijah had two husbands, Abu Halah Tamimi by whom she had a son named Hind bin Abu Halah, and Atiq bin Aidh Makhzumi, by whom she had a daughter named Hind. Then she was married to the Prophet (peace be upon him) and all the genealogists agree that from his loins she had the four daughters as stated above.” (see Tabari, vol. ii, p. 411: tabaqat Ibn Saad, vol. viii, pp. 14-16; Kitab al- Muhabbar, pp. 78, 79, 452; .9l-Isti'ab, vol. ii, p. 718) 
There are also assertions made by some Muslims that Prophet Muhammad ﷺ had only one daughter, Fatimah (r.a) through Khadijah (r.a), and the other daughters were not from his own blood but by the former husbands. Based on Quran and historical facts & sources of early Prophet’s biographers, these assertions are incorrect. The Quran specifically states that Prophet has daughters i.e. more than one: (Al Quran 33:59) “O Prophet, enjoin your wives and your daughters and the believing women, to draw a part of their outer coverings around them. It is likelier that they will be recognised and not molested. Allah is Most Forgiving, Most Merciful.” - This verse clearly states that Prophet has daughters and “your daughters” is not a reference to the females of the Ummah (Prophet’s nation) because the words “the believing women” is a reference to all Muslim females. Hence all four daughters namely: Umm Kulthum (r.a), Ruaqayya (r.a), Zaynab (r.a), Fatimah (r.a), Qasim (r.a) & Tahir (r.a), all six were born to Khadija (r.a) through Prophet ﷺ.
Maulana states that the Prophet’s earliest biographer, Muhammad bin Ishaq, after mentioning his marriage with Khadijah, says; “She was the mother of all the Prophet’s children except Ibrahim (who was from Maria Qibtiyyah). Prophet Muhammad’s (peace be upon him) children with Khadijah were Qasim, Tahir (Abdullah) or Tayyib, Zainab, Ruqayyah, Fatimah and Umm Kulthum.” (Ibn Hisham, vol. 1, p. 202). The famous genealogist, Hashim bin Muhammad bin as- Saib al-Kalbi states: “The first born child to the, Messenger (peace be upon him) of Allah before his call to Prophethood was Qasim, then Zainab was born to him, then Ruqayyah, then Umm Kulthum." (Tabaqat Ibn Saad, vol. 1, p. 133). Ibn Hazam writes in Jawami asSirah that from Khadijah the Prophet (peace be upon him) had four daughters, the eldest being Zainab, then Ruqayyah, then Fatimah and then Umm Kulthum (pp. 38-39)
Surah Al-Ahzab 33:59-68 - Towards Understanding the Quran - Quran Translation Commentary - Tafheem ul Quran - Maulana Maududi