It Was A Little Too Late For Both Girls - Tumblr Posts

At eight, the princess was placed into service as a cupbearer…but for her own father, the king. At table, at tourney, and at court, King Viserys thereafter was seldom seen without his daughter by his side. – Fire and Blood

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You will do, I think. For so long as I remain at Harrenhal, Nan, you shall be my cupbearer, and serve me at table and in chambers.“ This time she knew better than to say that she’d sooner work in the stables. “Yes, your lord. I mean, my lord.” – Arya IX, A Clash of Kings

The role of cupbearer is an apprenticeship for a leadership position for a child as it requires them to be at the side of a high ranking individual as they navigate their position of power. As cupbearers, Rhaenyra was present for audiences and council meetings where she would have learned politics and administration from her father, the king, as well as the others who were present. Similarly, when Arya became the cupbearer of Roose Bolton, she witnessed war councils and political meetings. This on the job training was specifically intended within the narrative to prepare Rhaenyra to become queen regnant, while GRRM intentionally includes this as part of Arya’s ongoing training.