In Reformed Theology how are baptism and circumcision of an infant comparable?

When talking to my generally reformed friends on the topic of pedo-baptism they often say that baptism is the new circumcision, circumcision was done on babies to bring them into the Old Covenant, therefore we baptize babies to bring them into the New Covenant. I have a hard time with this because before infant circumcision was not based on the faith of the child but on the parents and their adherents to God's command to do so. But in the New Covenant, this same theological truth does not apply. I can not come into the covenant unintentionally or outside of my will. I am sure Reformed Theology has an answer to this and I just have not seen it yet, so how would Reformed Theology answer this?