I went to medical school there, but graduated in 2005, so my info may not be totally current and I can't comment what it's like as a resident there. However the hospital is new and beautiful, and the faculty that I worked with there were all very nice. The volume is certainly excellent as well, so you'll see plenty.
Keep in mind that they used to have rotations down in the Bronx at another hospital, but many I knew felt that was a positive and different experience. Westchester Medical Center as a whole did have a lot of monetary problems when I was there, however I don't know if or how that affected the children's hospital.
Had I wanted to stay in the NY area, I probably would have stayed there, but I'm just not a fan of the northeast.