Are you referring to when children dress up as a “King or Queen” and act like they are reigning over their subjects?
It seems to be automatic in my games. I hear some music and the boy stands on a chair with a crown on and holding a scepter. He is also wearing a different shirt with a breast-plate. He waves the scepter around, claps his hands and basically acts like a King in Sims Medieval

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My child also has a moodlet for being 'King for a Day' but I didn't look to see how long it lasted. I haven't played any girl children yet so I am not sure how they look.