The 6D is in a different league than the other two. It has a completely different sensor (bigger) and thus is far more capable in low light. If you really have the money (don't forget you need lenses), then the 6D is the best pick for your uses. I have one, I also have 6 month olds and I'll tell you that it's a fantastic camera for little ones. However, I can, and do, make great photos of them with my old 450D Rebel. More important is your skill with both the camera, and the files it produces.
My recommendation wouldn't be any of those cameras. It'd be to get the latest Rebel, the 700D (or better, a refurbished 600D). With the money you save get a couple good lenses - you'll want several for kids. Later, once you've gained some experience you can reassess your camera.
I'll reiterate, because it's important: there's little point in getting a 6D if you're not going to invest in good lenses. Once the kids get mobile you'll need a telephoto lens, and for the 6D they start at about $1000, and goes up. If you can swallow that, fine, if not, the other cameras offer more affordable options.
As far as lenses. For a 8 month old you're going to want a fast prime for low light, somewhere around 50 - 85mm, and a macro lens for closeups of all those pudgy baby parts. Then a standard zoom for basic use. In a year you'll want to add a telephoto.
