There are those in Reformed theology that believe everything that happens in the universe is exactly at God ordains it, and thus the disobedience of Adam and Eve was all in the plan of God. This school of thought is called
Supralapsarianism. There are also
postlapsarianism view that somehow the election/predestination doesn't kick in until after the fall. I disagree with all this and it doesn't make sense to me.
Revelation describes war in heaven between God and Satan:
The tree of knowledge of good and evil, where the ancient serpent tempted and caused the downfall of our ancestors, was part of the battleground from the war in heaven. It was a test for loyalty. Would we take God at His word, and remain innocent of the knowledge of evil? Or would we listen to the devil, join his cause against God, and learn first-hand the difference between good and evil?
So it would not make sense for God to physically prevent our Adam and Eve from being allowed to choose sides. That was the point, "choice."
I reject the idea that God somehow wanted sin to come into our world. Instead, He wanted to spare us all the sorrow that would follow, and thus He warned our parents to stay away from it -- with a potential penalty of death. When they failed the test, God Himself took on the task of our salvation and redemption. When we break a bone in our body, cells leap into action to start the healing process -- but how much better would it have been if the break never occurred? Likewise God leapt into action to save us, but how much better if Adam and Eve had not rebelled in the first place?
tldr: Putting a cherubim as guard by the tree would have removed Adam and Eve's ability to choose.
Best wises,
KT