✅ Answer: First, regarding the method of the funeral prayer, please refer to the following link:

https://feghh14.com/en/1392

✅ Regarding the age of the child for whom the prayer is performed, please refer to the following link:

https://feghh14.com/en/967

✅ Now, pay attention to the following Hadith:

1 عَنْ عَلِیِّ بْنِ الْحُسَیْنِ عَنْ مُحَمَّدِ بْنِ یَحْیَی عَنْ مُحَمَّدِ بْنِ أَحْمَدَ بْنِ یَحْیَی عَنْ أَبِی الْجَوْزَاءِ الْمُنَبِّهِ بْنِ عَبْدِ اللَّهِ عَنِ الْحُسَیْنِ بْنِ عُلْوَانَ عَنْ عَمْرِو بْنِ خَالِدٍ عَنْ زَیْدِ بْنِ عَلِیٍّ عَنْ آبَائِهِ عَنْ عَلِیٍّ فِی الصَّلَاةِ عَلَی الطِّفْلِ أَنَّهُ کَانَ یَقُولُ اللَّهُمَّ اجْعَلْهُ لِأَبَوَیْهِ وَ لَنَا سَلَفاً وَ فَرَطاً وَ أَجْراً.

📚 Al-Tahdhib, Vol. 3, p. 195

Amir al-Mu'minin (peace be upon him) said regarding the prayer over a child, he says:

اللَّهُمَّ اجْعَلْهُ لِأَبَوَیْهِ وَ لَنَا سَلَفاً وَ فَرَطاً وَ أَجْراً.

✅ As observed in the Hadiths regarding the method of the funeral prayer, in most Hadiths, supplication (Dua) is made for the deceased after the fourth Takbir.

However, in the case of an immature child for whom the aforementioned supplications are not applicable, according to the statement of Amir al-Mu'minin (peace be upon him), the above supplication is recited. The other specific recitations and supplications narrated for the funeral prayer, such as the Shahadatayn (the two testimonies), supplication for the believing men and women, and sending peace upon the Prophets (peace be upon them), are recited in the exact same manner.

In fact, the above supplication is recited instead of the supplication for the deceased during the prayer over a child. Therefore, in Hadiths where supplication for the deceased is also narrated at times other than after the fourth Takbir, the above supplication is recited instead of those supplications for the deceased.

اللَّهُمَّ صَلِّ عَلَی مُحَمَّدٍ وَ آلِ مُحَمَّدٍ وَ عَجِّلْ لِوَلِیِّکَ الْفَرَجَ وَ الْعَافِیَةَ وَ النَّصْر

https://feghh14.com/en