I only shoot more or less static stuff with my x100 but I'd try the following to get sharper shots of moving kids:
1. Is it camera shake or subject movement causing the blurring? Make sure your shutter speeds are high enough. So many times I take blurry pics indoors, to find I am trying to handhold at 1/10th sec.
2. If it is subject movement then you could experiment with focussing methods. I cannot manually focus at all but I am sure it is possible for others. I don't use the AF-C setting on the x100 either but it might help. My AF approach would be to prefocus on a point and wait for the subject to get there.
3. Probably the easiest approach is to close down the aperture to get a deeper depth of field. With a wide angle lens like the x100's it doesn't take much to give you several feet of acceptable sharpness, especially if you are not too close to the subject. If it is dark, boost your ISO as noise is handled well on this camera and a slow shutter speed will give you subject blur. And don't pixel-peep - look at the shots full screen/at print size. Shots can looked blurred at 100% but are fine when viewed as intended.
4. Last thought is that the camera is so light I worry I cause camera shake when I take pictures. I understand a soft shutter release can help but I only managed about a dozen shots with mine before it fell off and I lost it. Make sure you are not the cause of the blurring though by shooting some static subjects and getting those sharp.