Prophet (SAW)'s Prayers When someone's child dies

Recite:

الْحَمْدُ لِلَّهِ إِنَّا لِلَّهِ وَإِنَّا إِلَيْهِ رَاجِعُونَ

This can also be read when any calamity befalls. By saying so, Allah instructs His angels to build a house for His servant in Jannah and to name it "Bait-ul-Hamd" (House of Praise). (Hisnul Hasin from Tirmidhi)