Oooh....peds psych can be really tough to start out with - those kids can really break your heart. But, if that's your passion, more power to you! I work with dual d/x, detox, straight psych, etc on the floor I'm on now. I absolutely LOVE my job!! I knew what I wanted to do before I even started nursing school, so I opted out of the med surg jobs after I graduated. I'll more than likely be in psych forever. :)
I know everyone says you need at least a year or 2 of MS under your belt, and I'm sure it couldn't hurt whatsoever...but I just didn't like it for myself. I do, however, have to do a little extra reading every now and then to try to keep up on the basics I learned in school and things that every nurse needs to know, no matter what type of work they are doing.
Hope that helps!! Good luck. :)