I have an XTi, so you have me beat with your original Digital Rebel.
One thing I'd say is that the high ISO performance on these cameras sucks. I have always been into nature and landscape, so it didn't matter. However now with my kids and shooting indoors more often, I find that even with my Sigma 30mm f/1.4 I need to bump the ISO to keep shutter speeds down or risk blurry images, and in turn some of the shots tend to have more noise.
What helped me with this was actually not on your list, but I'll throw in the suggestion: I got a an external flash (580EXII) and it's made a world of difference. I can get nice shots indoors without blowing out my kids faces with the on-camera flash, and haven't had to bump the ISO quite so high as a result.
So in your shoes, I'd almost say for the kids and family portraits, especially indoors, you'll benefit more from a flash than either a new lens or a new camera.
You'll rarely use a 70-200 indoors, especially with 1.6x crop factor on the Rebel. And the 24-70, while sounds like a great idea, will overlap with your 28-135 and you'll still have low light shooting issues when taking pics of the kiddos at home or at parties, etc.
Outside, in a park or something, I can see the longer lens being a plus, so it's a wash if you think you take 50% of your kids/family shots indoors vs. outdoors. So now I hope I haven't made it worse - new camera, new lens, or new flash? 