Whoever has a female child with him (under his guardianship from slavery), and trains her in the best behavior, and teaches her well, and then frees and marries her, will have a double reward.

faulty translation!

In this Hadith the Prophet was referring to raising one's own daughters till they're old enough to get married!

Sadly, before the advent of Islam pre-Islamic Arabs used to kill their daughters after birth.. so the Prophet said this to prevent ppl from killing their own daughters..