Supplication for the advancement of reward during the funeral prayer

اللَّهُمَّ أَعِذْهُ مِنْ عَذَابِ الْقَبْرِ ⁽¹⁾، اللَّهُمْ اجْعَلْهُ فَرَطًا وَذُخْرًا لِوَالِدَيْهِ، وَشَفِيعًا مُجَابًا، اللَّهُمَّ ثَقِّلْ بِهِ مَوَازِينَهُمَا، وَأَعْظِمْ بِهِ أُجُورَهُمَا، وَأَلْحِقْهُ بِصَالِحِ الْمُؤْمِنِينَ، وَاجْعَلْهُ فِي كَفَالَةِ إِبْرَاهِيمَ، وَقِهِ بِرَحْمَتِكَ عَذَابَ الْجَحِيمِ، وَأَبْدِلْهُ دَارًا خَيْرًا مِنْ دَارِهِ، وَأَهْلًا خَيْرًا مِنْ أَهْلِهِ، اللَّهُمَّ اغْفِرْ لِأَسْلَافِنَا، وَأَفْرَاطِنَا، وَمَنْ سَبَقَنَا بِالْإِيمَانِ ⁽²⁾.

This supplication is made when the deceased is a baby/child (i.e. one not having reached the age of puberty). After seeking forgiveness for the deceased, one can say: Allahummaj-AAalhu faratan, wathukhran liwalidayh, washafeeAAan mujaban. Allahumma thaqqil bihi mawazeenahuma wa-aAAthim bihi ojoorahuma, wa-alhiqhu bisalihil-mu/mineen, wajAAalhu fee kafalati Ibraheem, waqihi birahmatika AAathabal-jaheem.
‘O Allah, make him a preceding reward and a stored treasure for his parents, and an answered intercessor. O Allah, through him, make heavy their scales and magnify their reward. Unite him with the righteous believers, place him under the care of Ibraheem, and protect him by Your mercy from the torment of Hell.

اللَّهُمَّ اجْعَلْهُ لَنَا فَرَطًا، وَسَلَفًا، وَأَجْرًا ⁽¹⁾.

Al-Hasan used to recite the opening chapter of the Quran (i.e. AL-Fatihah) over the child and then supplicates: Allahummaj-AAalhu lana farata, wasalafan wa-ajra.
‘O Allah, make him a preceding reward, a prepayment and a recompense for us.