To the two posters praising Soujirou for being a proponent of gender equality in swordplay: believe it or not, that is actually historical. Iizasa Chōi-sai Ienao, founder of Tenshin Shōden Katori Shintō-ryū (the TL;DR full name of one of the oldest and maybe the most respected Japanese schools of martial arts) was well-known for being a radical (for his time) willing to transmit his teachings to peasants, women and (gasp) peasant women!
So having a little girl in Soujirou's dojo actually means that he follows the same school of thought regarding martial arts than one of the most respected Japanese swordman of all time. Subtle, but fits him 120%