Reference : Hisn al-Muslim 160

"اللهم أعذه من عذاب القبر "
وإن قال: 
"اللهم اجعله فرطاً وذخراً لوالديه ،
وشفيعاً مجاباً .
اللهم ثقل به موازينها 
وأعظم به أجورهما ، 
وألحقهُ بصالح المؤمنين ، 
واجعلهُ في كفالة إبراهيم ، 
وقه برحمتك عذاب الجحيم ، 
وأبدله داراً خيراً من داره ، 
وأهلاً خيراً من أهله ، 
اللهم اغفر لاسلافنا ، وأفراطنا ، 
ومن سبقنا بالإيمان " 
فحسن
Allāhumma a`idh’hu min `adhābi ‘l-qabr
[or say:]
Allāhumma ‘j`alhu faraṭan wa dhukhran liwālidayh, 
wa shafī`an mujāban.
Allāhumma thaqqil bihi mawāzīnahumā 
wa a`ẓim bihi ujūrahumā, 
wa alḥiqhu biṣāliḥi ‘l-mu'minīn, 
waj`alhu fī kafālati Ibrāhīm, 
wa qihi biraḥmatika `adhāba ‘l-jaḥīm, 
wa abdilhu dāran khayran min dārih, 
wa ahlan khayran min ahlih, 
Allāhumma’ghfir li aslāfinā wa afrāṭinā
wa man sabaqanā bil īmān.


O Allah, protect him from the torment of the grave. 
[It is also good to say:] 
O Allah, make him a precursor, a forerunner and a treasure for his parents and an answered intercessor. 
O Allah, make him weigh heavily in their scales (of good) and magnify their reward.
Make him join the righteous of the believers. 
Place him in the care of Ibrahim. 
Save him by Your mercy from the torment of Hell. 
Give him a home better than his home, 
and a family better than his family. 
O Allah, forgive those who have gone (i.e. passed away) before us, our children lost (by death), and those who have preceded us in Faith.

Reference:
Ibn Qudamah, Al-Mughni 3/416 and Ad-Duroosul-Muhimmah li-Aammatil-'Ummah, 
pg. 15, by Shaikh 'Abdul-'Aziz bin Baz.

Chapter: When the deceased is a child, during the funeral prayer