From a Calvinist perspective, if Adam refused the fruit what would happen to their descendants? And the woman saw that the tree was good to eat, and fair to the eyes, and delightful to behold: and she took of the fruit thereof, and did eat, and gave to her husband who did eat. Genesis 3:6


What would be the case for humanity if Adam refused the fruit after Eve decided to eat it?
The question of the descendants of Adam if there had been no fall has actually be discussed a great deal among Calvinists in the disciplines of Biblical and Covenant theology. This question is a lot less speculative than is immediately apparent.

The answer, in summary, is that Adam would have gained eternal life through his obedience and become glorified, and since the commission to have children was given prior to the fall, it would still have been in effect if there were no fall. Therefore he would have fathered children in a similar nature to his own. Since the commands concerning the Tree of Knowledge of Good and Evil were negative in nature, reformed theologians refer to an implicit doctrine of a "probationary period," in which Adam could prove his obedience and merit eschatological advancement: glorification. The need for this glorification is demonstrated in their nakedness (which in the salvation context is addressed by glorified clothing), and the fact that the Tree of Life is presented as being efficacious for this advancement.

Geerhardus Vos, the reformed biblical theologian, is known for coining the phrase, "Eschatology precedes soteriology." In short, with or without the fall, there needed to be a greater end-game for mankind, at a minimum to prevent the constant threat of a fall, but also to enjoy better communion with Jesus Christ. Soteriology became necessary due to the fact of the fall, but eschatology would have been present regardless. He addresses this topic directly in chapter 3 of Biblical Theology. There is an excellent podcast that discusses this question.

This is especially significant to understand the doctrine of the active obedience of Christ. If there were not a merit-based eschatological advancement available to one under the Covenant of Works, it becomes difficult to understand how exactly Jesus earned that advancement for Christians. He could be a perfect sacrifice and forgive our sins, but it doesn't make sense why that would bring us even beyond life in the Garden of Eden. Why be saved unto glorification instead of saved back into nakedness unless you factor in that Christ personally merited this through his obedience? This also explains some of the parallelism between Adam (the first man), the nation of Israel (God's firstborn), and Jesus (the only begotten), who was tempted by the devil in Matthew 4. The devil as his temper (like Adam was tempted) and in the wilderness (like the children of Israel). It also explains the "Tree of Life" references in the Bible in post-lapsarian contexts, Revelation 2 and 22. 

We also find references to a probationary period before Geerhardus Vos. 

Hermann Witsius (1677)


  ... That man was not arrived at the pitch of utmost happiness, but to
  expect a still greater good, after his course of obedience was
  over. This was hinted by the prohibition of the most delightful
  tree, whose fruit was, of any other, greatly to be desired; and this
  guard some degree of imperfection in that, in which man was forbid the
  enjoyment of some good. (via Puritan Board)


It is also in John Owen. (See same link above).

This is seen as the outworking of being faithful to the biblical text, not a speculation!