The first child who was born in the Islamic Land (i.e. Medina) amongst the Emigrants, was ʿAbdullah bin Az-Zubair. They brought him to the Prophet. The Prophet ﷺ took a date, and after chewing it, put its juice in his mouth. So the first thing that went into the child's stomach, was the saliva of the Prophet.
أَوَّلُ مَوْلُودٍ وُلِدَ فِي الإِسْلاَمِ عَبْدُ اللَّهِ بْنُ الزُّبَيْرِ أَتَوْا بِهِ النَّبِيَّ ﷺ فَأَخَذَ النَّبِيُّ ﷺ تَمْرَةً فَلاَكَهَا ثُمَّ أَدْخَلَهَا فِي فِيهِ فَأَوَّلُ مَا دَخَلَ بَطْنَهُ رِيقُ النَّبِيِّ ﷺ