We have no reason to believe that God amuses himself with fantasy. Does he daydream or ponder, "What if I were to..." trying to figure out what he might do before he did it? I don't think so. God either does or does not, and if he does, he understands what he does thoroughly. When God created, he would have known all conceptual things at once, and seen all historical things as he created. If he had not created, he would have seen and known nothing about creation.
Therefore, when there was no creation, God knew nothing of the acts of free agents, for there was nothing to know. When God created, he instantly knew exhaustively what was not in flux by the brute fact of his omniscience, and what was in flux from omnitemporal observation. Of course, something has to be in existence to be observed, so there is a distinction within the knowledge of God. What exists because of conception God knew when he conceived it, but what exists as a result of freedom he knows by creating that which can act in time and timelessly observing it throughout its time.
When God created the universe, he instantly knew its entire history, including that of mankind, because he observed it from a timeless vantage. The foreknowledge gained through omnitemporal observation is therefore exhaustive while the choices and acts of agents are free. The conception of children in Christ is the only template mentioned in scriptures which guided God in creating. We are never told in the scriptures that a conception of the damned burning throughout eternity, nor the precise acts of mankind, guided anything prior to creation.
There is an aspect of incrementalism in observational foreknowledge. For instance, when God said "let there be...", he would instantly know the history of all that existed in response to that decree. When he said, "let there be..." again, then he would know the entire history of what that decree brought into existence in conjunction with all the former decree had actualized. It is likely the former history would have been changed in some way by the latter decree. When God finished his creative work, the fullness of all he foreknows observationally would have been perfected.