According to premillennialism, are the nations in the New Earth still procreating? This question mainly deals with Revelation 21:24–27, but it is probably helpful to read from Rev 20 to the end also.  

Revelation 21 (KJV)  


  24 And the nations of them which are saved shall walk in the light of
  it: and the kings of the earth do bring their glory and honour into
  it.  25 And the gates of it shall not be shut at all by day: for there
  shall be no night there.  26 And they shall bring the glory and honour
  of the nations into it.  27 And there shall in no wise enter into it
  any thing that defileth, neither whatsoever worketh abomination, or
  maketh a lie: but they which are written in the Lamb's book of life.


I understand that these nations of the new earth are not procreating, however, there are supposedly people who are evil that are being kept out of the holy city at this time.  Where are they coming from if no one is having babies?   

You would think, that at this time all the people that are found in the Lamb's book of Life have eternal bodies, and the others are in the lake of fire.  Somehow these evil people in question should be in the lake of fire instead of being kept out of the gates of the holy city of the new earth, one would think.  The mere problem that sin is still with us in the new earth seems, well, weird.   

A good answer will be from the premillennial perspective, pre- or post-tribulation shouldn't matter.
In the historicist framework of Ellis Skolfield, Christ returns at the last trumpet (the seventh trumpet) and the time of the new Earth is immediately ushered in. The righteous receive their new bodies, while the damned are immediately dispatched to Hell. No one from this time forward has any children. In Matthew 22:29-30 it says: 


  Jesus replied, “You are in error because you do not know the
  Scriptures or the power of God. At the resurrection people will
  neither marry nor be given in marriage; they will be like the angels
  in heaven.


According to Skolfield, the tribulation and the millennial reign of Christ coexist simultaneously. Thus we are currently in the midst of the tribulation, and we are also in the Kingdom of God, because Christ ascended to the throne about 2,000 years ago.

As for the length of the time periods, he argues that the word for millennium used in the Greek, chillioi, is an indefinite plural, hence can refer to multiple thousands of years.

As for the time when the nations will once again be deceived, he argues that over the course of the last century, especially since the founding of Israel, many nations (faithful or not) that once gave lip service to Christ being their ultimate sovereign have since abandoned even the pretense of following him.