Meanmustard brought up some good points regarding the very possible mental capacity of Adam and Eve at this juncture, which I believe further explains and justifies the simplicity of God's command to stay away from the tree of knowledge.
Yes, but also their ability to ward off even the most rudimentary deceptions. Even their capacity to "trust" God is severely handicapped. More below...
The command was so simple and elementary so as to borderline on 'dumb'.
The simplicity doesn't have anything to do with it.
Why did the snake insist on eating from this forbidden tree given all the circumstances? As mustard pointed out, Adam and Eve couldn't have fathomed the consequences of disobedience, since even the concept of 'death' was not clear to them...such was their lack of comprehension.
But it goes much deeper. "Trust" is a "reliance on the character, ability, strength, or truth of someone or something". How can they rely on the "truth" of God when they don't even understand what "lies" are? What we would call someone's "character" is based on whether they believe and display "good" traits, and not "evil" ones - none of which Adam or Eve would be able to decern.
I don't even think trust works here - at least not any sort of "trust" to which you could attach moral culpability.
Adam could have wondered the garden, blissfully and innocently raping lions and goats, with his wife, and neither of them would be able to even begin to grasp why that be wrong. Those actions would hold the same moral weight as eating from the tree of knowledge of good and bad.
In other words, the ability to "trust" is still contingent on a sense of morality. What you are considering "trust" is more akin to blind obedience in the face of a contradicted threat, with no way to reasonably evaluate the sides - correction - without any awareness there are "sides" at all.