You have your work cut out for you but there is plenty of ammo. Start by giving them all of the scriptural verses that speak to baptismal regeration. I have listed them as follows:
And Peter said to them, "Repent, and be baptized every one of you in the name of Jesus Christ for the forgiveness of your sins; and you shall receive the gift of the Holy Spirit.
And he commanded them to be baptized in the name of Jesus Christ. Then they asked him to remain for some days.
While Apollos was at Corinth, Paul passed through the upper country and came to Ephesus. There he found some disciples. And he said to them, “Did you receive the Holy Spirit when you believed?” And they said, “No, we have never even heard that there is a Holy Spirit.” And he said, “Into what then were you baptized?” They said, “Into John’s baptism.” And Paul said, “John baptized with the baptism of repentance, telling the people to believe in the one who was to come after him, that is, Jesus.” On hearing this, they were baptized in the name of the Lord Jesus. And when Paul had laid his hands upon them, the Holy Spirit came on them; and they spoke with tongues and prophesied. There were about twelve of them in all.
And now why do you wait? Rise and be baptized, and wash away your sins, calling on his name.'
Go therefore and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit,
Jesus answered, "Truly, truly, I say to you, unless one is born of water and the Spirit, he cannot enter the kingdom of God.
He who believes and is baptized will be saved; but he who does not believe will be condemned.
ROMANS 6: 3-4
Do you not know that all of us who have been baptized into Christ Jesus were baptized into his death? We were buried therefore with him by baptism into death, so that as Christ was raised from the dead by the glory of the Father, we too might walk in newness of life.
For as many of you as were baptized into Christ have put on Christ.
In him also you were circumcised with a circumcision made without hands, by putting off the body of flesh in the circumcision of Christ; and you were buried with him in baptism, in which you were also raised with him through faith in the working of God, who raised him from the dead
one Lord, one faith, one baptism,
but when the goodness and loving kindness of God our Savior appeared, he saved us, not because of deeds done by us in righteousness, but in virtue of his own mercy, by the washing of regeneration and renewal in the Holy Spirit,
1 PETER 3:21
Baptism, which corresponds to this, now saves you, not as a removal of dirt from the body but as an appeal to God for a clear conscience, through the resurrection of Jesus Christ,
Once you have done this show them COLOSSIANS 2:11-12 cited above, and explain the connection Paul makes to circumcision. OT Jews circumcised babies on the eighth day after birth. Christian baptism replaces circumcision as entrance into the community of faith. The first time any dispute arose over infant baptism was approximately 200 to 300AD and the dispute was over whether it should be on the eighth day after birth. Once you have done this head to the following verses from the OT:
"Before I formed you in the womb I knew you, and before you were born I consecrated you; I appointed you a prophet to the nations."
For thou hast possessed my reins: thou hast covered me in my mother’s womb. I will praise thee; for I am fearfully and wonderfully made: marvelous are thy works; and that my soul knoweth right well. My substance was not hid from thee, when I was made in secret, and curiously wrought in the lowest parts of the earth.
These verses show the Lord’s view of us even before birth. Then immediately take them to Luke 1:15 which speaks of John the Baptist. It says, “for he will be great before the Lord, and he shall drink no wine nor strong drink, and he will be filled with the Holy Spirit, even from his mother’s womb.”
Finally, show them this verse from Luke 18:15-16 which says,
"Now they were bringing even **infants ** to him that he might touch them; and when the disciples saw it, they rebuked them.
But Jesus called them to him, saying, “Let the children come to me, and do not hinder them; for to such belongs the kingdom of God.”
Baptism is the sacrament of faith and faith is an unearned gift from God. Baptism removes original sin and brings us the gift of the Holy Spirit. An infant is entitled to this just as any adult is. You can do a little more to persuade them by pointing out that entire households were reportedly baptized in the NT such as those of the Roman jailer, Lydia, and Stephanes. Scripture does not indicate that children were not included.
I hope this helps.