Why will children born during the Millennium have no opportunity for open rebellion?
Because Satan is bound and no satanic force is operating, and Christ rules as King with absolute benevolent power. That external restraint removes the opportunity for open, outright rebellion, even though those children are still born with the same Adamic nature as their parents.
Les explains that children born during the Millennium from believers will have no opportunity for out-and-out rebellion because there is no satanic force operating and Christ will rule with absolute benevolent power as King. Nevertheless, they are still born with the same old Adamic nature as their parents. At the end, Satan must be released for a little season so this new generation, who never had to choose for or against God, can be brought to a place of choosing and exercise their free will. There must be more than one option, so Satan returns to present his options, just as he tempted Eve in the garden with the idea, “You can be like God.”