All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
Allaah has mentioned Thul-Kifl in the Qur'an in chapter Al-Anbiyaa' (the Prophets), Allaah says (which means): {And [mention] Ishmael and Idrees and Thul-Kifl; all were of the patient.}[Qur'an 21:85]. The interpreters of the Qur'an differed in opinion about whether he was a Prophet or just a righteous man.
In any case, it is a good name because it is either a name of a Prophet or a name of a righteous man.
Allaah knows best.