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
Pronunciation
Allahum-maj'alhu lanaa salafan, wa faratan, wa 'ajran
Meaning
O Allah, make him for us a precursor, a forerunner and a cause of reward.
Source: Mawquf Hasan (Albani). Ahkamul-Jana'iz, 160 Page
Duas from IslamicAPI's Masnun Dua dataset (based on the Quran, hadith, Hisnul Muslim and Rahe Belayat). See the dataset