A kunya for a child

Anas said, "The Prophet ﷺ visited us. I had a young brother who used the kunya of Abu 'Umayr. He had a sparrow which he used to play with it and it had died. The Prophet ﷺ came it and saw that he was sad. He asked, 'What is wrong with him?' He was told, 'His sparrow has died.' The Prophet said, 'Abu 'Umayr, what has happened to the little sparrow?'"

حَدَّثَنَا مُوسَى بْنُ إِسْمَاعِيلَ، قَالَ: حَدَّثَنَا حَمَّادُ بْنُ سَلَمَةَ، عَنْ ثَابِتٍ، عَنْ أَنَسٍ قَالَ: كَانَ النَّبِيُّ ﷺ يَدْخُلُ عَلَيْنَا، وَلِي أَخٌ صَغِيرٌ يُكَنَّى: أَبَا عُمَيْرٍ، وَكَانَ لَهُ نُغَرٌ يَلْعَبُ بِهِ فَمَاتَ، فَدَخَلَ النَّبِيُّ ﷺ فَرَآهُ حَزِينًا، فَقَالَ: مَا شَأْنُهُ؟ قِيلَ لَهُ: مَاتَ نُغَرُهُ، فَقَالَ: يَا أَبَا عُمَيْرٍ، مَا فَعَلَ النُّغَيْرُ؟.