This is an issue of dispute among scholars.
The hadeeth that was reported by the Bukhari, the Prophet salla Allahu alaihi wa sallam said: “With the boy there is his ‘aqeeqah so shed blood on his behalf and remove the harm from him”. The harm refers to the hair on the infant as explained in the other hadeeth: “The boy is in pledge for his ‘aqeeqah which should be slaughtered on his behalf on the seventh day, and he should be named and his head shaved”.
This “harm” refers to the hair which is mixed with different types of material that was surrounding the child while in his or her mother’s womb.
Therefore, the boy and the girl need such purification and removal of filth that sticks to their hair.