Thanks for the responses and advice. I totally understand what you mean about the grain in some photos but it looks worse when you open the image in full size. I've been working for 2 weeks straight (including nights) to edit and reduce the grain, and this is just one example of the best possible version for each one of the almost 200 photos. The reason why some images have grain is usually lack of appropriate lighting, something that happens quite often in reportage photography, is when the photographer tries to capture a moment that happens spontaneously, without focusing on posing their subjects or creating an ideal lighting setup.
During this shoot, the actions, poses and scenarios were very much led by the client, and I followed along with their vision without interfering too much, offering them the space to be themselves, capturing them in a way that felt natural and unforced. Personally, I don't normally do such spontaneous shoots. I follow quite a strict procedure that includes a clear shot list, and I move the furniture and lights around as necessary, to create an intentional frame and scene.
The good news is that the grain in the photos is almost invisible in print, and so is in the small version of each photo, when the client might use them online. Even the best photographers in the world produce grainy photos in low light situations. The main reason you see the grain so intense is mainly because of your large screen size.
Is there any reason why I can't deliver these images to the client as they are? I don't want to do any further work on this project.