As I mentioned above, blurry photos are usually due to a shutter speed problem. When you are taking photos of moving objects you need to make sure your shutter speed is high enough to freeze the frame. Even the slightest movement can cause blurriness at shutter speeds slower than 1/300. I typically shoot with a shutter speed of 1/500 or faster.
So now you may be asking, well how do you do that in lower light conditions? The secret is to raise your ISO. If you are using a specific aperture and a specific shutter speed then the only variable left to play with is ISO.
Well now you are saying, but raising my ISO is going to add noise or grain to my photos. While this may be true, most modern DSLR cameras will produce high-quality photos at the higher ISO values. Besides, the blur in the photo will be way more noticeable than any noise introduced by the ISO. I have shot professional photos with an ISO of 4000 or so but what stands out is that they are tack sharp and look crisp.
The second issue I mentioned above was a focal plane or aperture issue. When the aperture is wide open or close to wide open you have a very small focal plane in which the image will be sharp and in focus. Think of the depth of field in an image as cutting the image horizontally into different focal planes. When your aperture is closed down (higher number) the entire image and all the focal planes will be in focus. If your aperture is wide open (lower number) only certain sections of the image will be in focus and you need to make sure you have specifically chosen that part as the focal point in your camera. Let’s break this down with an image.
The focal point in the above image is on the girl in the front, which is why the girl behind her and everything else behind her looks blurry. The aperture for this image is f1.4. The solution to correct this problem is to either put both girls right next to each other on the same focal plane, or to decrease your aperture size (which is to say increase the f-stop value). Here is another example:
The red dotted lines are the “focal plane slices” if you were looking down and into the photo. Everything at the same depth of the focal point will be tack sharp (assuming your shutter speed is high enough). Anything in front of or behind that focal plane will be blurry and not tack sharp at this aperture of f2.8. But you can already see that the girl in the back is more in focus than the previous image because I decreased my aperture (increased my f-stop value). To get both these girls tack sharp in this position the aperture would need to be f7.1 or smaller (higher f-stop number). For a good explanation of depth of field and aperture, you can read this article: What is Aperture
To summarize, in order to get tack sharp photos you will need to have a high shutter speed and/or make sure you are using the proper aperture size for the number of people in your image. A shutter speed of 1/300 is probably the minimum you will want to shoot at and I usually only recommend that for adults who are standing still! Decreasing your aperture will also help create tack sharp photos because the focal plane you are working will be deeper, allowing more of the image to be in focus. Most important of all, practice makes perfect so keep that camera in your hand every day to improve and get the photos that you want.