The OPs idea is idiotic, but if we look in history, TW could really reconsider the age in which children can be married away or drafted into armies:
Mathilde, Daughter of emperor Otto II., was most likely 14 when she married her husband Ezzo.
Gisela, Daughter of Henry "the Wrangler" of Bavaria, was 10 when she married Stephan of Hungary, if the sources list her year of birth correct
Gisela of Swabia was 13 when she married her 1st husband Brun and 17 when she, now a widow, married her 2nd husband.
Eleanor of Aquitaine was 15 when she married first.
and so on... you could make a really long list. In middle age, it was common for noblemens children to marry really early. There is no reason TW couldn't lower the age children (or little brother and sister) become characters. Personally, I think 16 would be a good compromise between historical accuracy and modern moralistic thinking - Bannerlord is no middle-age simulation, and I don't want children marriages. Waiting 18 years to get main chars children playable just overextends my patience.