Message 4: Pastor Nichols refers to a sermon by Charles Spurgeon called Children Brought to Christ, and Not to the Font, which can be accessed here.
Message 31: As is mentioned, it was decided not to deal with this important aspect of the series in any great detail at this time. However, the "written notes" mentioned by pastor Nichols eventuated in his audio series The Covenants (and in a recent book). These materials are highly recommended, since a clear view of God's covenants is vitally important to properly understanding infant baptism.
Message 33: The paper by Walter Chantry which Pastor Nichols mentions can be accessed here.
Speakers: This series was delivered predominantly by Greg Nichols, but includes a handful of messages by Albert Martin (8-11, 22, 24, postscript in 31) & thus is listed under both names on each person's index page.
Associated series: Pastor Martin mentions that a related series has been "brewing" regarding how to deal with spirtually awakened children. This was eventually delivered, and can be found here.