Wow, what a loaded question! First off, let me just say that I've been there, done that within my own marriage, but instead of my in-laws being the problem, it was my own Catholic parents. Added to the fray was the fact that after I had been raised as a Catholic, albeit not a practicing one for a very long time, I had re-dedicated myself to the Lord and was thus baptized as an adult. Of course, my parents received my decision with bitterness and resentment because they believed that this second baptism was a move to convert myself from Catholicism to Protestantism - a very BIG "no-no" as far as they were concerned. Nevermind that I was attending a non-denominational Christian church, because it wasn't Catholic, it was WRONG.
Like the Greek Orthodox church, Catholics, especially Filipino-reared Catholics such as my mother and over-zealous converts, such as my father, are very much caught up in the tradition and nationality of being Catholic. Jesus trascends all nationality and tradition, and if you read the New Testament, especially the Gospels of Matthew, Mark, Luke, and John, you will discover that faith in Jesus as the Savior and acting upon that faith through love transcends ALL religious laws and traditions. Personally, when Jesus comes again, I want to be known as part of His church, not one so proudly based on cultural heritage and man-made traditions.
The first thing I would do before ever engaging in a spiritual discussion with others who aren't exactly on the same page as you is to PRAY. Pray for spiritual wisdom and discernment, and above all, pray for a loving and compassionate delivery of this wisdom.
Second, you need to decide what it is you really believe in. Whether you're a "born again," Orthodox, or Catholic, water baptism is the most important sacrament in Christianity because it SYMBOLIZES one's full, conscious, and PUBLIC acceptance that Jesus is the one and only Savior. Because Jesus himself was baptized as an adult in his 30s, many "born agains" take the lead from the Bible and feel called to be water baptized when they're adults...it doesn't have to be at a specific age, but the person must make a full and conscious decision to accept Christ as his/her Savior.
Growing up as a Catholic, I know that it was indoctrinated in parishioners' minds that baby baptism was somehow a one-way ticket to Heaven should a child ever die before adulthood. Catholic baby baptisms were also necessary to put the stamp of "Catholic" ownership on children's spiritual rearing...from that point on, my parents were always able to proudly say that I was baptized Catholic, however, not necessarily Christian. To Catholics and other Orthodox religions that are founded on man-made, doctrine-based laws and beliefs, this distinction is very, very important.
Dedication for "born agains" differ from baptism in the sense that it is not a sacrament. It certainly does not take the place of baptism because again, your child will need to make a conscious, adult, and public decision to accept Christ as his/her savior once he/she is ready. In Matthew 28:19-20, Christ himself gave his disciples all the authority in heaven through The Great Commission, which very clearly states to go forth and make disciples of all nations, baptizing them in the name of the Father, the Son, and the Holy Spirit and teaching them to obey everything He commanded.
You can see, therefore, that baptism is not merely a religious ceremony, but a pronouncement and acknowledgement of the ultimate truth: that Jesus is the Savior. Dedication is simply a testament that parents make to the body of the church about their INTENTIONS to raise their child in the spirit of Christianity according to the Word of the Lord. The hope is that the fruits of their labors will yield a child that grows up to know and love the Lord and then freely choose to be baptized in the presence of the church when called by the Spirit to do so. I'm not sure how long dedication ceremonies have been around in "born again" churches, but it seems to satisfy many of the orthodox religions' needs to have some sort of ceremony accompany the arrival of a new child.
Lest your in-laws get you caught up in a discussion/debate about the importance of following their religious laws, remind them that in Matthew 22:37-40, after silencing the Sadducees and Pharisees regarding the adherence of religious laws and tradition, Jesus' pronounced that the greatest commandments were to "Love the Lord your God with all your heart and with all your soul and with all your mind. This is the first and greatest commandment. And the second is like it: 'Love your neighbor as yourself.' All the Law and the Prophets hang on these two commandments."
Before you can ever hope to impress upon your in-laws why you feel it's important to wait for child's baptism, you must first understand your own faith. I recommend picking up the Bible study "Foundations - A Purpose-Driven Discipleship Resource" by Tom Holladay and Kay Warren. It focuses on the eleven core truths of Christianity: God, Jesus, The Holy Spirit, The Church, The Second Coming, Creation, The Afterlife, Sanctification, Good and Evil, the Bible, and Salvation. Rick Warren's "Purpose Driven Life" is also a fantastic study...I'm in fact doing it for the second time with a good friend, and it's even more relevant to me today. If you feel up to it, maybe you can do the Rick Warren study with your husband and your in-laws...it can easily be done through e-mail by reading a chapter a day and answering the very short, but reflective questions at the end of each chapter. The book can be found in any bookstore because it is the best-selling non-fiction hardback book in history. Nothing, of course, beats the Bible. That should be your first and primary resource with regards to seeking out spiritual guidance on anything and everything.
Christians should never shy away from learning and maturing spiritually, no matter how old they are or how much they think they know about their faith. If your in-laws are citing religious respect over Godly duty as a reason to baptize your child now, then they clearly need to re-think the meaning and focus of their faith.
To give you some good news, once my parents were able to see how God transformed my life, they started to back off on their demands to have my children baptized as Catholics. I remained faithful and prayed, and they ultimately came to realize and accept that the most important thing to consider was that my husband and I were raising our children to know, obey, and love the Lord - a very hard thing to do in today's liberal society. I never thought my parents would come around from their staunch, religious beliefs, but they did and it wasn't through me...it was through the grace of God. I assure you that getting through this tough time with your in-laws is possible, but it starts with you and your husband first. God bless you and your family...I will definitely pray for you all.