Reward for praising Allah and saying 'Inna lillahi wa inna ilayhi raji'un' upon the death of a child.

The reward is that Allah (Exalted be He) will build a house for him in Paradise and name it 'the House of Praise'. The Prophet (peace be upon him) said: 'When a servant's child dies, Allah says to His angels: "Have you taken the soul of My servant's child?" They say: "Yes." He says: "Have you taken the fruit of his heart?" They say: "Yes." He says: "What did My servant say?" They say: "He praised You and said: 'Inna lillahi wa inna ilayhi raji'un' (To Allah we belong and to Him we shall return)." Then Allah says: "Build a house for My servant in Paradise and name it the House of Praise."' (Ahmad).