لَّوْ أَرَادَ ٱللَّهُ أَن يَتَّخِذَ وَلَدًا لَّٱصْطَفَىٰ مِمَّا يَخْلُقُ مَا يَشَآءُ سُبْحَٰنَهُۥ هُوَ ٱللَّهُ ٱلْوَٰحِدُ ٱلْقَهَّارُ
"Had God willed to take a child, He would have chosen what He willed from among what He creates. He is far above that. He is God, the One, the Qahhār."
The sentence is built with the conditional law (if), and in Arabic this particle conveys a condition that has not come about.
I record this as my own reading, and its ground is the structure of the sentence: before rejecting the claim outright, the verse shows the claim's outcome within the claim's own logic — had there been such a thing, the one chosen would still have been one of the created. That is, the notion of "child" cannot step outside the category of createdness.
ٱصْطَفَىٰ — root ص-ف-و: to purify, to choose by sifting. That the root lies in the same field as ikhlāṣ is worth recording: the first three verses of the sūra were built with the word khāliṣ (خ-ل-ص), and here ص-ف-و comes. Both roots carry the picture of separating out from a mixture.
ٱلْوَٰحِدُ ٱلْقَهَّار — these two names were treated together in Ghāfir (al-Muʾmin) at 40/16. I rely on that treatment.