Originally posted by Flickeroo I'm just a recent hobbyist as well, so my suggestions might be wrong.
I see low-light situations: I suggest a fast lens (F2.8 or below)
I see close focusing: it looks like you're 5 or 6 inches away on some of these pictures
I see very wide-angles: you probably need something under 50mm
You could go for either a zoom lens or a prime. My guess since you're starting you might want the zoom to give yourself a bit more selection.
I tried thinking of your budget of under or around $1,000 for both the K-x and Lens, and here are my suggestions:
Prime: SMC Pentax-DA 35mm F2.8 Limited Macro
Zoom: Tamron SP AF17-50mm F/2.8 XR LD ASPHERICAL (IF)
Of course, I don't have either of these lenses but I wish I did!
Flickeroo
An aperture sub-2.8 lens will not help you at all for low-light, moving subject shots. The thinner the DOF, the harder it is for the image to stay sharp, and especially difficult for AF to function correctly. This is triply so for AF-C.
You need higher ISO, and/or a bounce flash, especially to stop action.
The point of ISO is vary the speed of the image based on the amount of light. The purpose of the aperture is primarily to set the DOF. If you try and use a wider apertures (usually below 2.8) then you start to encounter the above-mentioned difficulties and the larger aperture actually starts to work against the other elements (like AF) necessary to track the subject and make an acceptably sharp exposure.
I highly recommend the Tamron 17-50 + any flash that can bounce. For your subject matter, these are critical.