Photographing mobile small children + 18-140mm zoom range sounds good to me. My Mrs talked me into getting an 18-200mm. I was resistant as I wanted something faster/sharper like a 17-50mm f/2.8. Begrudgingly, she was right, as its amazing how two toddlers playing outside can force you to use a huge zoom range. Switching between an 18-55 kit and something longer (I had kit 55-200mm) is impossible with my little terrors for subjects. The 17-50mm is still on the list of "wants" but many decent photos have been captured at long focal lengths thanks to that 18-200mm lens (I think 18-140 will do the trick too).
Have you got a "roadmap" or even a loose idea of lenses you want to acquire as you go forward?
This could make a difference which way you want to go. I'll limit these examples to Canon, Pentax and Nikon DSLR (without making any brand a recommendation, including those not mentioned).
For example, the Canon 17-55mm f/2.8 (referred to by Richard) and the Pentax 16-50mm f/2.8 are almost half the price of the Nikon 17-55mm f/2.8, at least in Australia. I'm not sure if that's true in other parts. If you head the more budget route with Sigma or Tamron, then this is a moot point.
Another example might be if you do your street photography with an unobtrusive prime? The budget friendly options are the Canon EF-S 24mm F/2.8, Pentax 35mm f/2.4 or the Nikon 35mm f/1.8. Some might rightly argue 35mm or even 24mm isn't wide enough on APS-C size sensor, but it's an example of the budget prime offerings and how you get some difference between the companies.
I can't make any recommendation. I've just thought about it and researched it a bit as I'm eventually going to have to go through the same process as you, from the different perspective of the Nikon equivalent (or close to) of the 1100D (the D3000).
One final comment is that if you are already settled on the Nikon range, you are right to strive for the D7xxx. This avoids a few issues of the D3xxx and D5xxx range for a portrait photographer.
1. The lower level cameras d3xxx and d5xxx can't do HSS (auto-FP sync in Nikon language), regardless of the flash you use. This is important for flash with fast shutter speed in outdoor portraiture.
2. The on camera flash can't act as a master flash for remote optical TTL flash triggering.
3. They lack an internal focus motor which limits the lens selection that will auto focus on these bodies.