Thanks chaps - Addy (the youngest) is developing quite a little personality and look, as she edges towards her 2nd birthday - still hits the house like a tornado though!
This was my first go shooting super low light and moving targets with the X100, and I'm very happy with the results. As Hamish pointed out, it's ISO6400 so we have some grain, which has a decent look to it in B&W, and the shots are super sharp at f2.
AF struggled a little in the very low light, with lock taking a couple of attempts in some cases - I played with the size of the AF area (a nice feature on the Fuji) and that seemed to assist the AF lock. Being able to place the AF area pretty much anywhere in the field of view was very useful, and allowed easy off center compositions with accurate focus every time.
The camera is basically silent, so the grandkids played away without really reacting to 'having their photo taken', and not using the flash also kept the intrusion to a minimum.
The live view on the rear screen made composition easier when holding the camera down at kid level, so grandpa didn't have to actually get on the floor to get his shots!