A child once they become Bar/Bas Mitzvah is now an adult and is obligated in all the mitzvos (assuming they've reached physical maturity). In the case where they became Bar Mitzvah on Shabbas (Friday night after dark), if the family took Shabbas in early and made Kiddush before dark, after dark is the Bar Mitzvah obligated (or even permitted) to repeat Kiddush?
In other words, was the Kiddush that was made for them before dark considered a דרבנן, and now they are obligated מן התורה? What about the brachos, as those are only דרבנן? What is the solution to this situation? (Besides finding another family who hasn't said Kiddush yet to be motzi).
The Mishnah Berurah addresses taking shabbas in early not working to make a child an adult, so that's not a solution.
משנה ברורה סימן נ"ג סקל"ג
במקומות שנוהגין להתפלל של שבת בע"ש מבעוד יום משום תוספת שבת מ"מ הרי עדיין לא נשלמו לו י"ג שנה עד ליל שבת שתוספת שבת אינו מועיל לענין שנות ימי הנער