Du'a for children's affairs

اللَّهُمَّ يَسِّرْ أَمْرَهُمْ وَوَفِّقْهُمْ فِي حَيَاتِهِمْ

Allahumma yassir amrahum wa waffiqhum fi hayatihim

O Allah, ease their affairs and grant them success in their lives.

A general supplication, not a narration from the Prophet (peace be upon him).