JOEL - First of all, telling another person what is wrong with their religion is probably not the best way to go about discussing the gospel with them. It tends to put them on the offensive and you find yourself in a debate that goes nowhere. The better attitude is to just explain to them the reasons why you believe in LDS doctrines.
Here are a few scriptures and thoughts on this subject that might be helpful.
Baptism of infants was not known at all for the first 200 years after Christ set up His Church. The first mentioning of infant baptism was not until around 240 AD by a Greek theologian named Origen who argued that baptism takes away the polution of birth brought on by the "original sin" of Adam. Catholics believe that infants are born into the world carrying with them the guilt of the so-called sin of Adam. Because of this they must be baptized as an infant to remove this sin, otherwise they would be sent to hell when they die. However, Adam was inocent in the garden and was incapable of knowing right from wrong, and therefore could not sin, even though his transgression brought the potential for man to sin into the world. Therefore infants are not born into the world carrying any sin at all with them and therefore need no baptism
In fact the scriptures say:

"Except ye ... become as little children, ye shall not enter into the kingdom of heaven." (Matt 18:3)

A child is born into a sinful world and has within him the potential to sin later in life (because of Adam's transgression), but in order to sin a person has to be able to make a conscience decision to disobey a commandment of God; something infants and small children simply cannot do. And I think most Catholics agree with this.
Scriptures speaking of baptism associate it with faith in Jesus Christ, repentance, and forgiveness of sins, none of which infants are capable of doing (Mark 1:4-5; 16:15-16; Acts 2:37-38; 19:4; 22:16; Rom. 6:1-6; 1 Cor. 6:9-11; Gal. 3:26-27; Col. 2:12-13; Heb. 6:1-6; 10:22; 1 Pet. 3:21) .
Jesus Christ said of children, "Suffer little children, and forbid them not, to come unto me: for of such is the kingdom of heaven." (Matt. 19:14)
Requiring infant baptism is as though we are forbiding them to enter the kingdom of heaven unless they are baptized. What happens if they die before anyone has a chance to baptize them?
We all suffer from the effects of Adam's transgression but we are not responsible for the transgression itself.
In Acts we read:

"Then Peter said unto them, Repent, and be baptized every one of you in the name of Jesus Christ for the remission of sins, and ye shall receive the gift of the Holy Ghost.
For the promise is unto you, and to your children, and to all that are afar off, even as many as the Lord our God shall call. (Acts 2: 38, 39)

Notice that verse says that we must "Repent, and be baptized". Since infants are incapable of sin they cannot nor need not repent, and therefore need no baptism. Mormon explained this best in his letter to his son Moroni in the Book of Mormon:

"For, if I have learned the truth, there have been disputations among you concerning the baptism of your little children.
Listen to the words of Christ, your Redeemer, your Lord and your God. Behold, I came into the world not to call the righteous but sinners to repentance; the whole need no physician, but they that are sick; wherefore, little children are whole, for they are not capable of committing sin; wherefore the curse of Adam is taken from them in me, that it hath no power over them; and the law of circumcision is done away in me.
And after this manner did the Holy Ghost manifest the word of God unto me; wherefore, my beloved son, I know that it is solemn mockery before God, that ye should baptize little children.
Behold I say unto you that this thing shall ye teach—repentance and baptism unto those who are aaccountable and capable of committing sin; yea, teach parents that they must repent and be baptized, and humble themselves as their little children, and they shall all be saved with their little children.
And their little children need no repentance, neither baptism. Behold, baptism is unto repentance to the fulfilling the commandments unto the remission of sins.
But little children are alive in Christ, even from the foundation of the world; if not so, God is a partial God, and also a changeable God, and a respecter to persons; for how many little children have died without baptism!
Wherefore, if little children could not be saved without baptism, these must have gone to an endless hell.
Behold I say unto you, that he that supposeth that little children need baptism is in the gall of bitterness and in the bonds of iniquity; for he hath neither faith, hope, nor charity; wherefore, should he be cut off while in the thought, he must go down to hell.
For awful is the wickedness to suppose that God saveth one child because of baptism, and the other must perish because he hath no baptism." (Moroni 8: 5-15)

And in our Doctrine and Covenants:

"And I also beheld that all children who die before they arrive at the years of accountability are saved in the Celestial Kingdom of heaven." (D&C 137:10)

To be fair, Catholics do believe that God will do something for infants who die without baptism to keep them out of Hell, but they have no doctrines to explain how this will happen and at the same time remain in harmony with their original sin doctrine.

Another aspect of Catholic baptism is the manner in which it is done; sprinkling rather than by immersion. LDS baptize by immersion, following the example set by Jesus who was baptized by John the Baptist.(Matt 3:13-17) Here are a few scriptures that support baptism by immersion:

"And Jesus, when he was baptized, went up straightway out of the water: and, lo, the heavens were opened unto him, and he saw the Spirit of God descending like a dove, and lighting upon him:" (Matt 3:16)

"And it came to pass in those days, that Jesus came from Nazareth of Galilee, and was baptized of John in Jordan.
And straightway coming up out of the water, he saw the heavens opened, and the Spirit like a dove descending upon him:" (Mark 1:9-10)

In order to come "straightway out of the water", one must start by being in the water.

"Therefore we are buried with him by baptism into death: that like as Christ was raised up from the dead by the glory of the Father, even so we also should walk in newness of life." (Rom 6:4)

Being placed down into the water (immersed) during the baptism ordinance represents our bodies being placed down into the grave after death as explained by Paul above; and then the rising out of the water represents the resurresction, where our bodies reunite with our spirits and rise up out of the grave, the same as Christ did. It also represents the candidate's new birth to a life in Christ, being born of God, thus born again of the water and of the spirit.

"And John also was baptizing in Aenon near to Salim, because there was much water there: and they came, and were baptized." (John 3:23)

If baptism were to be done by sprinkling and not immersion, why would they need "much water" to do it?

Of course much of what we know about the time and proper manner of performing the ordinance of baptism has come by modern revealtion to our prophets, which Catholics would not recognize.

Link to Scripture