I am new to photography and have a new Nikon D40X which I love and am just about getting to grips with. I have the 18-55mm kit lens, and the f/1.8 50mm prime lens (which I am struggling with as I have to manually focus - I didn't do my research properly before buying that one!) but now feel as though I need a lens with more zoom. I like taking natural candid photos of my kids but if I get too close I get the false cheesy grins - I want to be able to sneak pics of them without them noticing. Also, I like photographing animals/insects etc but they usually end up as dots in the middle of the photo as I can't get close enough to them without scaring them away.

Santa says I can have a new lens for Christmas and I have been looking at the 18-135mm and 18-200mm lenses. There seems to be a huge difference in the price but I don't really understand how they differ in what they can do. Can anyone give me some guidance please? Thank you for your time.