Something just doesn't seem right. The picture on the right is from the camera held vertically? There is no background light in that case, there are plenty of mid tones, and as long as you weren't on spot metering, that jiuce box shouldn't have thrown off the multi-segment metering that badly.
Something is set that we are not thinking about. Were any pictures from this event fine, or were they all this badly underexposed? No one can tell me that this is typical of the k10d.
One more thought...Did you check what the exposure values were for these pictures? With the original jpeg we could see what all the values were in the EXIF (fstop, aperture, ISO). I just realized you used the kit lens and were inside, and did not use a flash. Is it entirely possible that this is just an underexposed pic beyond the limits of the camera with that lens? Was either the aperture or shutter speed values flashing when you metered? When I am indoors I don't even bother taking out my zoom. I use my 50/1.4.
Oh yeah, I thought of something else...what is your auto ISO limit? You may have that set so that your camera could not auto adjust the iso high enough for an exposure indoors. The default is something like 400.
All questions we could answer if we could see your original EXIF...
Can you post the original jpeg (not saved out of Photoshop) of the right side picture? I really think we can solve this if we know the exposure settings and if you can tell us the general lighting of the room (dimly lit, windows only, etc.). You are absolutely right in expecting that your $800 k10d should have nailed that picture when just about any P&S could. (Heck the reason many of us have DSLR's is to get the best possible pics of our kids!) It's just that the k10d has some default settings that you may need to change for the way you shoot. The P&S no doubt would have fired the flash, but with the proper settings, the k10d may have been able to capture a much warmer shot without flash.