Michael, there are many things that Christians accept as common practice that are not explicitly in the bible. For instance, that God is a Trinity - Father, Son and Holy Spirit; this is not explicitly stated in the bible and yet this has been a teaching of Christianity since the time of the Apostles. When scripture tells us in Acts 16:33 (RSVCE) "And he took them the same hour of the night, and washed their wounds, and he was baptized at once, with all his family." we can safely assume that Paul does exactly what he states, that he baptizes the entire household since all are worthy of entrance into the Church. If Baptism saves us (as scripture says) then why would we withhold God's saving grace from an infant?