Traditionally, it is said that Sukayna bint al-Husayn and Ruqayyah bint al-Husayn were 2 daughters of Imam Husayn who were present in Karbala. (In some cultures, they are treated as one and called "Sakina").
From a historical perspective, it can be deduced from historical reports that Sukayna bint al-Husayn was a young woman (maybe about 13 years old) at the time of Karbala, and that this is the same Sukayna bint al-Husayn who was mentioned in later historical sources. Also, in a poem ascribed to Imam Husayn to comfort her, he refers to her as the "best of women" (that is, she wouldn't have been a young child or else he wouldn't have called her a "woman"):
سيطول بعدي يا سكينة! فاعلمي * منك البكاء إذا الحمام دهاني لا تحرقي قلبي بدمعك حسرة * ما دام مني الروح في جثماني وإذا قتلت فأنت أولى بالذي * تأتينه يا خيرة النسوان
"O, my dear Sukayna! Know that after me your weeping is prolonged. [So my daughter] do not burn my heart by your sorrowful tears as long as I am alive. O, the best of women! Weeping is more suitable for you after my martyrdom."
It is said that Ruqayyah was about 3 years old in Karbala, that she died when Yazid sent her the head of her father, and that her shrine is in Syria. However, historical reports about her are scant. Maybe people in those days didn't consider it too important to keep records about young children as children often did not live past childhood.