Posting up a picture with teh EXIF intact would help people give you better advise, rather than just guessing about the camera settings or the picture. If you don't want to post pictures, at least describe the situation (lighting condition, kids' activity/motion, etc) and the camera settings (shutter speed, aperture, ISO, AF mode, focal distance, etc)
Short of more info from you, here's my speculation:
Try to switch your autofocus mode to AF-C (continuous AF).
If you were in AF-S (single AF) mode, with a very active/fast kid, especially if he is running towards or away from you, between the AF lock and the picture being taken, he may have run outside your focus depth of field, so he will be out of focus.
And you may be working with a very narrow/thin depth of focus. This is because with a high shutter speed, your aperture is going to be big, so thinner DOF. And if you zoom in, the more you zoom in, I think the thinner the DOF too.
Don't go too high with your shutter speed. Try around 1/200 plus or minus first to see if that works.
I prefer a capture where I can get the face sharp, but there is some motion blur with the hands or feet. Gives a nice sence of motion.