Description of the du`a for the deceased in the Janaazah prayer
Question

How should we make du`a in the funeral prayer if it is offered for a male or a female or for two people, or if the deceased is a child?.
Praise be to Allah.Du`a should be made for all the deceased, whether males or females, by saying “Allahumma aghfir lahum warhamhum… (O Allah forgive them and have mercy on them…)”. If they are two you should say, ‘Allahumma aghfir lahuma warhamhuma… (O Allah, forgive them both and have mercy on them both…)”. 
If the deceased is a child, what should be said in the du`a is: “Allahumma aj’alhu dhikhran li waalidayhi wa faratan wa shafee’an mujaaban. Allahumma a’zim bihi ujoorahuma wa thaqqil bihi mawaazeenahuma wa alhaqhu bi saalih salaf al-mu’mineen waj’alhu fi kafaalat Ibraaheem ‘alayhi’l-salaam, wa qihi bi rahmatika ‘adhaab il-jaheem (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 Ibraaheem, peace be upon him, and protect him from the torment of Hell by Your mercy).”