Ash-Shaykh Shaykh Bin Baaz rahimahullah was asked the following question,
“What is the Du`a’ (supplication) that one should say during the Funeral Prayer for a child?”
“The Funeral Prayer for a child is not different from that offered for the adults. However, upon making Du`a’, one should say,
اللَّهُمَّ اجْعَلْهُ فَرَطَاً وَذُخْراً لِوَالِدَيْهِ، وشَفِيعاً مُجَاباً، اللَّهُمَّ ثَقِّلْ بِهِ مَوَازِيْنَهُمَا، وأعْظِمْ بهِ أُجُورَهُمَا، وألْحِقْهُ بِصَالِحِ الـمُؤْمِنينَ، واجْعَلْهُ فِي كَفَالَةِ إِبْرَاهِيمَ، وَقِهِ بِرَحْمَتِكَ عَذَابَ الجَحِيمِ
“Allahumma ij’alhu dhukhran li walidayhi wa faratan wa shafi’an mujaaban. Allahumma a’dhim bihi ujurahuma wa thaqqil bihi mawazinahuma wa alhiqhu bi salih salaf al-mu’minin waj’alhu fi kafaalat Ibrahim ‘alayhi al-salam, wa qihi bi rahmatika ‘adhab il-jahim
(O Allah, make him a stored treasure for his parents, and a preceding reward and an answered intercessor. O Allah, increase their reward through him and make their scales weigh heavily. Join him with the righteous believers who have gone on before, place him in the care of Ibrahim (peace be upon him) and protect him from the torment of Hell by Your mercy).
It was authentically that the Prophet (peace be upon him) said:
Funeral Prayer should be offered for the dead child, asking forgiveness and mercy for his parents.“
End quote. [Reference]
A Side Note:
Ash-Shaykh Albany rahimahullah would see that the following prophetic du’a should be given preference since it includes of making du’a for the little ones (i.e children).
اللَّهُمَّ اغْفِرْ لِحَيِّنَا وَمَيِّتِنَا وَصَغِيرِنَا وَكَبِيرِنَا وَذَكَرِنَا وَأُنْثَانَا وَشَاهِدِنَا وَغَائِبِنَا ، اللَّهُمَّ مَنْ أَحْيَيْتَهُ مِنَّا فَأَحْيِهِ عَلَى الْإِيمَانِ ، وَمَنْ تَوَفَّيْتَهُ مِنَّا فَتَوَفَّهُ عَلَى الْإِسْلَامِ ، اللَّهُمَّ لَا تَحْرِمْنَا أَجْرَهُ ، وَلَا تُضِلَّنَا بَعْدَهُ
Allahummaghfir lihayyina wa mayyitina, wa shahidina wa gha’ibina, wa saghirina wa kabirina, wa dhakarina wa unthana. Allahumma man ahyaitahu minna faahyihi ‘alal-Islam, wa man tawaffaytahu minna fa tawaffahu ‘alal- iman. Allahumma la tahrimna ajrahu wa la tudillana ba’dahu.
[O Allah, forgive our living and our dead, those who are present and those who are absent, our young and our old, our males and our females. O Allah, whomever of us You cause to live, let him live in Islam, and whomever of us You cause to die, let him die in (a state of) faith. O Allah, do not deprive us of his reward, and do not let us go astray after him].
However, along with the above du’a it is encouraged that the person makes du’a for the parents of the deceased child.
Ash-Shaykh Abdul-Muhsin al-Abbaad hafidaho-Allah was asked the following question:
هل يأخذ الطفل حكم السقط في الدعاء لوالديه بالمغفرة والرحمة؟
“Does the deceased child take the ruling of a baby that died from a miscarriage with making du’a for his parents to be forgiven and shown mercy upon?”
الجواب: الذي يبدو أنه كذلك؛ لأنهما بمعنى واحد.
“That is what is apparent since both have the same meaning.” End translation. [Sharh Sunan Abu Dawud # 367]
AbdulFattaah Bin Uthman