The word Fitrah has various meanings. In the Hadith you mention, Fitrah means such a natural disposition in which is vested the quality of accepting the trust. Thus, if the child had to grow up with no outside influence whatsoever, naturally, the person would turn towards the worship of one Allah. (al-Nihaayah vol.2 pg.379; Darul Ma?arif)
In another Hadith, Nabi [Sallallaahu Alayhi Wasallam] mentions, ?Ten things are from the Fitrah; a) To trim the moustache, b) To lengthen the beard, c) To make Miswaak, d) To put water into the nose, e) To clip the fingernails, f) To pluck the underarm hair, g) To shave the pubic hair, h) To use water after relieving oneself.
The narrator of the Hadith, Mus?ab ibn Shayba [Radhiallaahu anhu] states, ?I forgot the tenth one, but it might have been rinsing the mouth.? (Muslim Hadith196; ibn Katheer)
In this Hadith, the word ?Fitrah? means from the Sunnah, i.e. the Sunnah of the prophets who we have been instructed to emulate. (al-Nihaayah vol.2 pg.380)
and Allah Ta’ala Knows Best
Mufti Ebrahim Desai