It has been a source of frustration that of late Pentax has found itself unable to provide review samples to Gordon. Maybe that will change now that Ricoh are taking over the camera side at Pentax but that doesn't help you any. The forum search engine struggles with short search strings like "K-x" but fortunately Google can come to the rescue as you can see by clicking here
. There's actually a thread comparing the K-x with the D3100 but it can sometimes be worth bearing in mind that not every opinion in such threads is offered by someone who owns, or even has used, the models under discussion. While we don't often offer links to other review sites I think it would be churlish in this instance not to refer you to this K-x review
. Having used none of the cameras you mention I'll have to decline the invitation to pass comment apart from noting that you'd have to have a strong reason to prefer Pentax over the Nikon as, obviously, with the Pentax you'd not be able to share lenses with your son.
What do you think your priorities are for your new camera at, or near, your target $400 budget? You've mentioned size and there cameras with reasonably large sensors, such as the G1, which are pretty small. But, to really throw the cat amongst the pigeons, do you actually need a DSLR or will a high end compact, with manual adjustment of aperture, shutter speed and ISO number allow you to share photography as a hobby? Maybe Gordon's article Should you buy a DSLR or a Compact?
will be an interesting read? Most DSLRs offer good, or even great, video these days so that might be worth bearing in mind as you read the article but it may not be so true of the older models.