Disclaimer: I'm speaking as a nurse who started in an adult CVICU, and that's still where I am. Obviously our hospitals' orientation programs and environments differ, as we don't even do transplants at my hospital and I'm guessing y'all don't do open heart surgeries in the transplant ICU But, I know that even though I didn't die of anxiety going to work after being there for a year, I know SO much more now just 6 months later than I did then. And I am sure in June, when I will have been there for 2 years, I'll look back and realize I've learned even more since then.
However, if you are currently working with adults, but you want to do peds, it's gonna be a big transition regardless if you have 1 year or 2 years of experience, given that the pathophysiologies you'll be dealing with differ so much between the two population types. If PICU is your ultimate goal, and you are not bound by a contract, I don't think you will be doing yourself a big disservice by transferring after a year. If you were wanting to switch from one adult ICU to another, I would definitely recommend staying the 2 years as the skillset you acquire will more than likely transfer without issue. I can't say the same for going from an adult ICU to a pediatric ICU, regardless of the type.
Perhaps someone who has worked both can chime in on the benefits of 1 year vs 2 years.