What is the ruling on pronouncing "عليكم" (upon you) and "عليهم" (upon them) with a kasrah on the letter lâm, and does this affect the prayer, especially for one who is not attentive?

Breaking the lam in "عَلَيْهِمْ" (ʿalayhim) and "عَلَيْكُمْ" (ʿalaykum) when reciting the Qur’an is a لحن (lahan - a phonetic mistake/deviation from proper recitation). If one intentionally does this in prayer, their prayer is invalidated because they are deemed to be trifling (with the recitation/prayer).