"If I were a ten year old girl I would run among the flowers."
Translation:אם הייתי ילדה בת עשר הייתי רצה בין הפרחים.
I sorta think that too, but I realize I don’t know.
Is there a grammatical rule for counterfactual gender? If a man says “If I were a girl, I would run”, should he say “הייתי רץ” according to his actual gender, or should he say “הייתי רצה” according to the gender he would have in the imagined scenario?
If you say that, the translation is "If I were ten, I would....". Although I see what you are getting at, that the בן/בת difference gives you more information than you get in the English sentence.
But equally, saying אני קוראת ספר tells you "I (a female) am reading a book", and you don't translate the part that tells you the gender.