I enjoyed living in Seattle, although I don't know that I'd want to live there permanently due to the high cost of living. Even though the hospitals are unionized, the pay is still notoriously pretty low relative to the high cost of living. When I worked in Seattle my salary was $10/hr higher than my job in the South, but in the South I made about $20,000 more per year in savings (after tax) than I did in Seattle because of my living expenses.
I haven't worked in the NICU at Children's, but I've known and worked with several people who do. It's a great place to learn; you'll see cases you won't see anywhere else, and you'll learn many skills that most NICU nurses don't have (i.e. managing ECMO patients, CRRT, etc.). However I will caution you that they do have a higher burn-out and turn-over rate than some other regional NICUs. You may or may not know that Children's does not have it's own delivery facilities; they are referral only, therefore the NICU takes only the sickest babies. Most NICUs are in hospitals that do deliveries; therefore, in addition to the handful of super-sick kids, the vast majority of the babies are only a bit premature. Deaths in most NICUs are very rare; deaths in a NICU like Children's are more common. In most NICUs, the babies stay on the unit until they go home, even when they're not actually 'sick' (they're just learning to eat); you get to watch your kids get better and go home. In contrast, the Children's NICU only has intensive care patients; when the kids start to get better, they go to step down or the floor. Therefore, you may not do as much feeding, holding, or cuddling because the kids simply aren't stable enough. Most NICUs are a mix of intensive care and 'baby med-surg' patients, whereas a place like Children's tends to have more true intensive care. In addition, Children's has relatively few preemies, and far more term kids with congenital anomalies. With Level IV experience at a NICU like Children's, you can work or travel just about anywhere in the future. However, realize that it is a very different environment from most NICUs, and that it may be more high-stress (both emotionally and professionally).