I found it helpful to walk into a store and actually get the cameras in my hands. Any Target or Best Buy will have something for you to get your hands on.
FWIW, I went from point and shoot ignorance to an EVIL camera this year and have had good results. I don’t know what you’re doing for software, but I’d recommend budgeting for lightroom 3.
dpreview.com is a pretty good reference.
If you take one thing from Ken Rockwell’s site, it should be this: the photographer matters more than the camera. Nearly any DSLR that the average photographer can buy will be capable of shooting professional level photographs if the photographer understands what he is doing. If not, then even the most expensive camera will take mediocre shots.
Photography is a lot like fishing: it is mostly about being in the right place at the right time, and having good technique. The gear matters a lot less.
The biggest advantage of higher end cameras is things like built in off camera wireless flash functionality which allow for fancier lighting effects using a flash, and the Nikon D3100 doesnt have the ability to be used with an Infrared remote or have wireless flash caability, although the 3000 does. The 3000 however doesnt have the ability to use optional GPS units, while the 3100 does.
Its these kinds of less obvious features rather than the sensor that you’re more likely to see on higher end or newer cameras, with the other big feature of newer cameras being video as an option. The other advantage of higher end cameras is usually more direct access to manual functions, eg changing aperture and shutter speed with separate controls.
If your main focus is children and outdoor sport though, they wont be so important, and your focus should probably be on lenses rather than on the camera body.
Otara
You forgot Olympus, they partnered with Panny for the first gen of EVIL cameras.