From the book “Taribeeyatul-Awlaad fee Dhaw’il-Kitaab was Sunnah” (Educating the Children in the Light of the Book (Qur’aan) and the Sunnah), introduced by Ash-Shaykh, Al-Aalim, Ar-Rabbaaniyy (the educator) Saalih inb Fawzaan Al-Fawzaan (May Allah preserve him upon good) and prepared by Abdus-Salaam Ibn Abdillah Sulaymaan (May Allah preserve him upon good).
So the first of what the child begins with in speech, obligatory upon al-waalidayn (the father and mother), is teaching him Kalimatut-Tawheed, and to instruct to it. Ibn Abbaas (May Allah be pleased with his father and him) narrates that An-Nabiyy (the Prophet) (Allah’s peace and blessings upon him) said: ((Establish upon your children the first statement, Laa Ilaaha Illallah and instruct them at the time of death Laa Ilaaha Illallah)).