With this chain of narration from Al-Husayn ibn Sa'id, from 'Uthman ibn 'Isa, from Sama'ah: He said:
I asked Imam عليه السلام about the urine of a child that touches the garment. Imam عليه السلام said: 'Wash it.'" I said: 'What if I cannot find its exact location?' Imam عليه السلام said: 'Wash the entire garment.'"
Ali Akbar al-Ghaffari
Tahdhīb al-Aḥkām fī Sharḥ al-Muqniʿah, vol. 1, p. 283