Regarding the marital status and children of Li Jiancheng, here’s a general overview.
At the time of his death during the Xuanwu Gate Incident in the ninth year of the WuDe Era, Li Jiancheng was thirty-eight years old. Calculating backward, he would have been twenty-nine when the Tang dynasty was founded. As the legitimate eldest son and heir of the Sui dynasty's Duke of Tang, it is almost certain that he was already married by that age. Given the prevalent custom among noblemen of that era to keep multiple concubines, it is also hard to imagine he had no children by then.
However, official historical records mention his sons rather late. According to the "Old Book of Tang, Annals of Gaozu I": (In the sixth month of the third year of WuDe) The emperor enfeoffed his sons Yuanjing as Prince Zhao, Yuanchang as Prince Lu, and Yuanheng as Prince Feng; his grandsons Chengzong as Prince of Taiyuan, Chengdao as Prince of Anlu, Chengqian as Prince of Hengshan, Ke as Prince of Changsha, and Tai as Prince of Yidu. Among them, Li Chengzong and Li Chengdao were Li Jiancheng’s eldest and second sons. Yet, at the founding of the dynasty in the first year of WuDe, Li Yuan had already enfeoffed distant clan relatives and deceased illegitimate sons. If he had grandsons at that time, it is highly unlikely he would have waited two years to enfeoff them.
From this, it can be inferred that Li Jiancheng’s legitimate sons, like those of Li Shimin, were born after the Tang dynasty was established. As for the birthdates of his daughters, the only known record is the epitaph of his second daughter, Li Wanshun, who was born around the fifth year of WuDe. Thus, her only elder sister was likely also born after the Tang’s founding.
Li Jiancheng’s principal wife, Zheng Guanyin, according to her epitaph, entered the Crown Prince's Palace only after the Tang’s founding, unlike Empress Zhangsun, who married Li Shimin during the Sui dynasty. The epitaph states: "Our Gaozu, like a dragon lurking in the river, secretly foreshadowed the destiny of songs; the eldest son of the Crown Prince, residing in the east, was to inherit the glory of the throne. The wild goose mat was prepared, and the river bream was chosen. The lady’s speech and demeanor matured early, her hair pinned up in tender youth; as the ivy flourished, she was led in a light carriage to the imperial court. Married into the great state at the age of sixteen, she shone like the small stars, singing of the Three and Five."
This raises a question: If Li Jiancheng’s wife and children all appeared after the Tang’s founding, did he live as a monk before the age of twenty-nine? A more plausible explanation is that when Li Yuan raised his army in Taiyuan, many of his relatives and clan members were captured by Sui officials and generals. Some of these "rebel family members" were executed, such as Li Yuan’s fifth son, Li Zhiyun. Li Jiancheng’s principal wife and children likely all perished during this time. The lives of women and children were deemed insignificant, and for well-known reasons, they left no trace in historical records.
To add a bit more, according to the prevailing laws of the time, the actions of the officials in charge were illegal. Even for those guilty of the "Ten Unforgivable Crimes," which warranted the extermination of entire families, only adult males closely related to the principal offender were executed, while young children and female relatives were merely enslaved. For example, one Sui official named Yin Shishi, who killed Li Yuan’s relatives and desecrated his ancestral graves, is recorded in the "Book of Sui." After Li Yuan captured Chang’an, he retaliated by exterminating the Yin family. However, as the victor, he could afford to show magnanimity—Yin Shishi’s daughter and a young son, Yin Hongzhi, were spared and enslaved in the Prince of Qin's Manor. Yin Hongzhi later fathered Li Shimin’s fifth son, Li You, and even rose to the position of "Chief of Imperial Equipage," stirring up trouble as the uncle of a prince.
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