Why not p. 1379 of the Roman Missal:

F. For the Funeral of a Child
Who Died before Baptism
Should a child whom the parents wished to be baptized, die before Baptism, the Diocesan
Bishop, taking into consideration pastoral circumstances, may permit the funeral to be
celebrated in the home of the deceased child, or even according to the form of funeral rites
otherwise customarily used in the region.
In funerals of this kind there should ordinarily be a Liturgy of the Word, as described in
the Roman Ritual. Nevertheless, if at times the celebration of Mass is judged opportune, the
following texts should be used.
In catechesis, however, proper care is to be taken that the doctrine of the necessity of
Baptism is not obscured in the minds of the faithful.
Entrance Antiphon Rev 21: 4
God will wipe every tear from their eyes,
and there shall be no more death
or mourning, crying or pain,
for former things have passed away.
Collect
Receive the prayers of your faithful, Lord,
and grant that those you allow to be weighed down
by their longing for the child taken from them
may be raised up by faith to hope in your compassion.
Through our Lord Jesus Christ, your Son,
who lives and reigns with you in the unity of the Holy Spirit,
one God, for ever and ever.
Or
O God, searcher of hearts and most loving consoler,
who know the faith of these parents,
grant that, as they mourn their child,
now departed from this life,
they may be assured
that he (she) has been entrusted to your divine compassion.
Through our Lord Jesus Christ, your Son,
who lives and reigns with you in the unity of the Holy Spirit,
one God, for ever and ever.
Prayer over the Offerings
Graciously accept this offering, O God,
as a sign of our devotion,
so that, trusting in the designs of your providence,
we may be raised up by your gentle and fatherly care.
Through Christ our Lord.
Preface for the Dead, pp. 622-631.
Communion Antiphon Is 25: 8
The Lord God will destroy death for ever.
He will wipe away the tears from every cheek.
Prayer after Communion
Having received the Communion
of your Son’s Body and Blood, O Lord,
we ask you faithfully
to comfort amid the sorrows of this life
those whom you have graciously nourished
by these sacred mysteries,
so as to strengthen their hope of life eternal.
Through Christ our Lord

Attached are the relevant pages from Fr. Weber's Volume 4 of The Proper of the Mass, which is available for download for free.

That was what was traditionally used in the preconciliar ritual for *baptized* children that died before attaining the age of reason, but the current Missal already has its own Order of Christian Funerals.

I've shared it before, but in the event that you still need a psalm: