Praise be to Allaah.  

Du’aa’ should be made for all the deceased, whether males or females, by saying “Allaahumma aghfir lahum warhamhum… (O Allaah forgive them and have mercy on them…)”. If they are two you should say, ‘Allaahumma aghfir lahuma warhamhuma… (O Allaah, forgive them both and have mercy on them both…)”. 

If the deceased is a child, what should be said in the du’aa’ is: “Allaahumma aj’alhu dhikhran li waalidayhi wa faratan wa shafee’an mujaaban. Allaahumma 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 Allaah, make him a stored treasure for his parents, and a preceding reward and an answered intercessor. O Allaah, 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).” 

Kitaab Majmoo’ Fataawa wa Maqaalaat Mutanawwi’ah li Samaahat al-Shaykh ‘Abd al-‘Azeez ibn Baaz, 13/145.