I'm fortunate enough to have my grandchildren close and they are frequent subjects. One lens that has consistently delivered my best results for family photography is my Nikon 17-55mm f/2.8 AFS DX lens. This lens is also compatible with the D7000.
The 17-55mm is now considered a mid-range zoom. Something that qualifies for general photography (family/events/travel/etc.). This lens is a super quality lens and could be considered a DX Pro lens. It is a f/2.8 aperture meaning it lets in a lot of light for indoor photography. The f/2.8 aperture also means that you have some aperture control for DOF - however your 50mm f/1.4 is best for this. The 17-55mm is a heavy and somewhat bulky lens.
Another good choice is the 16-85mm f/3.5-5.6 AFS DX lens. This is top-rated consumer lens - meaning that it doesn't have the constant f/2.8 aperture. The longer reach of this lens over the 17-55 would make it a slightly better choice for travel, but the smaller aperture selection would restrict it a bit more for natural low-light photography. The 16-85mm is a bit more compact than the 17-55 f/2.8 and might be preferred as a carry everywhere lens because of that.
The cost difference between these two lenses is significant. Depending on your needs, one might be a better choice over the other (regardless of cost). It's a good exercise to take a few evenings and review your images to see if any kind of usage pattern emerges with your current lenses. I've always found that information valuable for making new lens choices.
BTW: I'm sure you would find either of these lenses perform better than your current 18-200.
There are a lot of possibilities and the right lens is a personal decision. I hope this helps bring a few things to light. Good luck in your selection.