I wondered what happened to the second page...
Eh, it's all about the details.
1. Under exposed
2. Over exposed
3. Tilting horizon.
4. Distracting background.
I don't have any great PP skills but I use Adobe Elements version 6 (it's up to v8 which I have seen on sale for about $60). There is also the free
FastStone Image Viewer, Screen Capture, Photo Resizer ... image viewer/editor among others.
With Elements, it's very easy to fix 1, 2 and 3 with almost just a single click, and perhaps (a) if you shoot RAW - moving sliders or (b) if you shoot jpgs - changing numerical values...adjust the brightness and contrast to see what works best.
Even removing background clutter in 4 is not too difficult. And it can lighten shadows, etc.
Howzabout a similar pose in the front or back yard with a prettier portion of your house as the background? (The house is not the subject, necessarily, but a decent background). Use a white sheet on the ground or draped on a chair out of the shot in front of the kids to bounce some light up and open the shadows. Try different f-stops. The smaller the number (larger the aperture) will help make the background a little softer.
And finally, use the center point to autofocus (I think you can set that though you can not see the focus point in the viewfinder on the K-x) by pressing the shutter 1/2 way, but then recompose the shot so that their heads are not dead center with way too much sky above them. Try to balance the mass in the frame - and watch the corners for trashcans :-)
Here's a quick edit - hope you don't mind.