The famous Tabi'i Hasan Basri (R) used to recite Surah Fatiha at the janaza of child and recite the following du'a:

اَللَّهُمَّ اجْعَلْهُ لَنَا سَلَفًا وَّفَرَطًا وَّأَجْرًا

Allahum-maj'alhu lanaa salafan, wa faratan, wa 'ajran

O Allah, make him for us a precursor, a forerunner and a cause of reward.

Reference: Mawquf Hasan (Albani). Ahkamul-Jana'iz, 160 Page