I'm DMing a recently-started campaign with 5 new players. They are completely new to RPGs and roleplay. In a combat, they often phrased their attacks in ways such as "I aim at the eyes in order to blind him and shoot my arrow", or "with a swift dash forward, I try to pierce her leg with my rapier so to incapacitate her movements", etc., and expected me to make the enemies suffer the consequences of their detailed actions, especially when they rolled high on the d20.
We are all enthusiasts of roleplaying and narrating and we enjoy describing events in detail. They thought they had a wider range of freedom for various or creative actions in fight, in order to affect the outcome of an attack outside of the loss of HP. However, I tried to explain to them that such an approach in combats can prove to be obnoxious and indeed the rules themselves just plainly avoid the topic.
I managed to convince them to abandon the idea of such complicated mechanics, but they were still disappointed about the side effect - so to speak - on roleplay. They wondered what the point of combat narration is anyway and started to unhappily phrase everything as such: "I attack him", "I throw [spell] at him". This resulted in coldly mechanical fights and dull narration, despite my efforts. I said I won't retract my stand on the topic; it can be quite hard to rule about anything and it's just a spit away from one-shotting an enemy due to a critical hit "aimed at the neck" and I don't want to deal with the balance issues.
However, I don't know how to flavor my battles and involve them in the narration without doing so. I mean, if they come out with a specific move and it succeeds, let's say with a critical hit, what do I say indeed? "Yeah, they lose their leg, but since we don't apply any mechanical effect to that, they still move as normal"? It feels odd. Frankly, I don't have enough experience for this. I wish to have some advices on how to narrate battles.
What can I do to keep my players engaged in combat narration without making it fall into a headshot-seeking madness? How can I encourage roleplaying in fights without allowing crippling and state-inflicting wounds (such as arrows into the eyes) to be made?