Reference : Hisn al-Muslim 161

اللهُـمِّ اجْعَلْـهُ لَنا فَرَطـاً، وَسَلَـفاً وَأَجْـراً
Allāhumma’j`alhu lanā faraṭan, wa salafan, wa ajra.

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

Reference:
Al-Hasan (Al-Basri) used to recite Surat Al-Fatihah for a child's funeral and then say this. Al-Bukhari, Kitabul-Jana'iz, p. 65.

Chapter: When the deceased is a child, during the funeral prayer