I know that most hospitals keep pediatrics separate from nursery because most pediatric patients are considered "dirty". One would never want pediatrics near newborn nursery, due to infection control issues. For the same reason, pedi should be set aside from nicu as well. Most hospitals, in my experience, are keeping l&d/nursery/nicu together since they are clean. Pedi and picu can be in the same area, since most picu are dirty. However, pedi/onc should be in a clean area, as their defenses are down during treatments.
Hope that helps. If anyone has corrections, just add in. I've worked in mostly rural hospitals and this is my experience.