https://www.catholicdoors.com/prayers/english5/p03283.htm
PRAYER FOR A
DECEASED CHILD.
To You, O Lord,
we humbly entrust this child,
so precious in Your sight.
Take him/her into Your arms
and welcome him/her into paradise,
where there will be no sorrow,
no weeping nor pain,
but the fullness of peace and joy
with Your Son and the Holy Spirit
forever and ever.
Amen.
and to pray for those bereaved by the death of their child:
O Lord, whose ways are beyond understanding,
listen to the prayers of your faithful people:
that those weighed down by grief
at the loss of this little child
may find reassurance in your infinite goodness.
Through Christ our Lord.
R/. Amen.
from: https://www.philadelphiacatholiccemeteries.com/blog/catholic-prayers-for-grief