Is the story mentioned above, about a child who has the phrase "Convey from me, even if it is a single verse" appear on his back, and then faces tribulations because of it, true or could it be so?

The truth of the story cannot be ascertained, and there is no great benefit in researching it. A Muslim should not preoccupy himself with it, as it may be a figment of imagination intended to divert Muslims from their religion. What is required of a Muslim is faith and righteous deeds, for they are the cause of victory, in accordance with the Almighty's saying: "O you who have believed, if you support Allah, He will support you and plant firmly your feet." This is a promise from Allah to grant victory to those who support Him by upholding His religion, inviting others to it, and striving against His enemies.