Generally speaking, the greater the range of a zoom, the more of a compromise the lens is, so the 17-50 is likely to be better than the 18-140. And that's quite apart from the 17-55 being a significantly faster lens in terms of aperture. My money would go on a good, fast short zoom - the Tamron 17-50 seems to get decent reviews and also has (IIRC) a 5 year warranty if bought new and UK stock. At that short a length and especially given its intended use, I would stick with the non VC version since VC will be of little use in keeping children still and for landscapes a tripod (or similar support) is a good idea and would render VC redundant anyway!
ETA Kit lenses are generally built down to a price rather than up to a quality. They usually do the job but once you use better lenses you'll feel and see the difference. Having said that, the old Nikon 18-70 kit lens was a very good performer especially for the price. Probably getting a bit long in the tooth these days though and isn't really fast enough for low light use (in terms of aperture rather that AF).
Edit to correct the focal length range of the 17-50 (NOT 55!)
