Du'a for children's purity

اللَّهُمَّ اغْفِرْ ذَنْبَهُمْ، وَطَهِّرْ قَلْبَهُمْ، وَحَصِّنْ فَرْجَهُمْ

Allahummaghfir dhanbahum, wa tahhir qalbahum, wa hassin farjahum

O Allah, forgive their sins, purify their hearts, and protect their chastity.

Adapted into the plural for children from Musnad Aḥmad 22211.