All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
This Hadeeth includes every child, be it living or dead, young or old, married or unmarried; as there is no evidence, to the best of our knowledge, to specify this merit for a particular child apart from others.
For further information about things by which the living can benefit the dead, refer to Fatwa 84938.
Allaah Knows best.