Where is that in the Bible?
Every instance of Baptism that is described in the New Covenant period follows salvation. This is demonstrated in various ways (the convert heard the gospel and believed, the convert repented, they confessed their belief, etc), but there is not one instance of a baptism occuring before conversion, and not one instance where a child is mentioned as being baptized.
My opinion is that, unless your child is old enough to understand what the gospel message is and confess their belief, they are not old enough to be baptized.
Based on biblical example, baptism should follow as soon as possible after conversion. Minutes, or perhaps hours, would be a normal time line.