While in WDW the first week in May I found myself shooting a lot with an 8 month old on my left arm and the camera in my right. The T2i doesn't make me feel too sure handed.
Got it - that's helpful. Ergonomics are one of those things that noone can give perfect advice on since everyone's hands and feel are different...but I can understand from what you're saying that a deeper, more pronounced grip would work better for you (I have always disliked Canon ergonomics myself because I tend to feel more comfortable with larger, deeper grips for one-handed carrying).
I also have a problem seeing the entire scene through the viewfinder without smashing my eye against it.
This may or may not get too much better with the other cameras you are mentioning - they all have roughly the same sized viewfinder, with roughly the same dimensions, magnification, and only small differences in eye relief - so I'm not sure you'll do a whole lot better. The Nikons have a slightly more pronounced eyecup, and the Sony A55 which uses an electronic viewfinder that is much larger to view also has a very extended viewfinder eyepiece which might help. But I'd expect very small differences overall with most.
Anyway, I dont have an investment in lenses so thats not a problem. My brother brought a Nikon D3100 with us and the grip feels good with one hand.
If that's the way your hand tends to feel comfortable, then probably Canon is the worst models for you to look at...they have very small, very shallow grips. Pentax has a more ergonomic grip design that sticks out a bit more than Canon, with a thin camera body and slightly squared off on the right side flat edge. Nikon has a fatter body with deeper but more rounded grip. Sony A55 has a deep but narrow grip on a smaller body. Sony A580 has both a fatter body and a very deep grip.
I have the 18-135 and the focus is horrible in low light sometimes never getting focus.
That is going to be the fault of the lens - a better lens would both focus better and give better images in low light, on any camera. Though the Pentax, Sony, and Nikon all share the same Sony-made sensor which does slightly outperform the older sensor in your T2 for high ISO shooting, the kit lenses will still not really give you the best representation of what those cameras are capable of. Eventually if you intend to really take a lot of these types of shots, you may want to look at a faster, better lens.
I dont like the res. screen on the 3100.
Look at the resolution figures for the LCDs on the cameras you are comparing - many of the cheaper or older entry level cameras have a low, 200-400K res screen...some of the newer models and a few others typically pair a much nicer 900+K screen.
How big is the Sony in comparison to the K-r, I dont know anything about the A580.
The A580 is larger overall than the KR - it's a medium sized body - it is similarly fat and chunky in body design to the Nikon D5000, but it also has a deeper grip with a larger fingerwell. The design is somewhere in between something like the T2 cheaper entry-level models, and the D7000 enthusiast-level models - it's sort of an 'advanced' entry level camera. The 16MP sensor in the A580 is made by Sony, and is also used in the Nikon D5100, D7000, Pentax K5, and Sony A55 - it is widely considered to be one of the finest low light sensors you can get.
The Sony A55, on the other hand, is smaller and lighter than the other cameras here - even slightly smaller than the KR - it does still have a deeper grip design which Sony/Minolta has always favored, but was designed from the outset to be smaller than DSLRs.
