Fortress of the Muslim (Hisn al-Muslim)
حصن المسلم
(56) Chapter: When the deceased is a child, during the funeral prayer
(56) الدعاء للفرط في الصلاة عليه
Hisn al-Muslim 161 
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.
اللهُـمِّ اجْعَلْـهُ لَنا فَرَطـاً، وَسَلَـفاً وَأَجْـراً
Reference : Hisn al-Muslim 161