What is the fate of the soul of a child who died after falling from the fourth floor, and did he know his demise was near?

We hope your son is in Paradise under the care of Ibrahim (peace be upon him). The children of Muslims are the butterflies of Paradise; one of them will meet his father and take hold of his garment until Allah admits both of them into Paradise. This is evidence that Muslim children are in Paradise. As for his knowing of his death and the interpretation of his words, we have no knowledge regarding that, except that the end of one's term (of life) is known only to Allah the Almighty, for it is part of the unseen.