At the end of the seven year time of Tribulation, Christ will return with His saints. The saints who return with Jesus will have their new glorified bodies (Philippians 3:21). Those who survived the Tribulation Hour and who are not condemned in the judgment of nations are allowed to enter the Millennium in their mortal bodies (Matthew 25:34). Since they are in mortal bodies and are able to procreate, babies will be born during this thousand year period. They, of course, will also have mortal bodies.
Because Satan is bound in Revelation 20:1,2), there is no enticement to sin among these mortals, their children, grandchildren, and great grandchildren. However, the children born during the Millennium still possess the old Adamic, or sin prone, nature. Because Christ rules with a rod of iron, they are kept under control for that thousand year period.
After 1,000 years of being bound, Satan is loosed again for a brief season (see Revelation 20:7); because God has always wanted everyone to follow Him by his or her own free will. So these millennial children are tested, and I am sad to say that multitudes after being with Jesus and seeing His sweetness, His tenderness, His love and His compassion will say, I want to be with Satan. It is really the same as at His first coming, when the people were with Jesus, and yet refused Him.
The Great White Throne Judgment is described in Revelation 20:11-15. This judgment is only for unbelievers. The unsaved dead are resurrected from the temporary Hell (Hades), stand before God to be judged out of the books mentioned in Revelation 20:12, hear that their names are not found in the book of Life because they have never received Christ, and are cast ALIVE into the permanent Hell known as the Lake of Fire (Gehenna) to be eternally tormented and separated from God.
After the one thousand years of the Millennium, there will be a renovation, a purging through fire. II Peter 3:10 says The Day of the Lord will come as a thief in the night; in the which the heavens shall pass away with a great noise, and the elements shall melt with fervent heat, the earth also and the works that are therein shall be burned up. This means literally that the earth shall be purged with fire. This happens because when Satan is released from the pit, he will gather a multitude of people who will turn against Christ. Punishment will come and the earth is purged with fire, including heavens one and two, and that is why Revelation 21:1 says I saw a new heaven and a new earth: for the first heaven and the first earth were passed away. This is the purging through fire, but the globe still stays here because it is going to go on for ever.