What is the Ruling of a Father who doesn't Command his Children to Perform Salah?

Yes. The father who doesn't command his children to (perform) Ṣalāh is sinful, and likewise is their brother, paternal uncle, maternal uncle, mother; all of them are sinful if they neglect (this). Enjoining the good and forbidding the evil is obligatory. Therefore, It's mandatory upon them to enjoin good and forbid evil, and it's from the obligations of the believers. He, Taʿāla [The Exalted] says: "The believing men and women are Auliyā’ (supporters) of one another; they enjoin (on the people) Al-Ma‘rūf (i.e. Islāmic Monotheism and all that Islâm orders one to do), and forbid (people) from Al-Munkar (i.e. polytheism and disbelief of all kinds, and all that Islām has forbidden)." [Sūrah At-Tawbah, 71] And the Prophet ﷺ said, "Whoever sees Munkar among you, then let him change it with his hand, if he's unable to, then, with his tongue. If he's unable, then, with his heart." They're capable of speech, meaning, they're capable of condemning through speech. Therefore, the mother condemns, (likewise does) the father, elder brother, paternal uncle, maternal uncle, and others. For indeed, this is from the ways of helping one another in righteousness and piety. Allāh, Glorified be He, said, "Help you one another in Al-Birr and At-Taqwā (virtue, righteousness and piety)." [Sūrah Al-Māʾidah, 2] And He, Glorified be He, said, "By Al-‘Asr (the time). Verily, man is in loss, except those who believe (in Islâmic Monotheism) and do righteous good deeds, and recommend one another to the truth and recommend one another to patience. [Sūrah Al-ʿAṣr, 1-3] It's a must to help one another in righteousness and piety. There's no doubt that condemning the one who abandons the prayer is from the ways of helping one another in righteousness, piety and a daʿwah (calling) to Allāh, the Exalted.