I spent a week in a sarcoma wing of a hospital. There were a lot of different types of scans used--PET, MRI, CT, and maybe some others that I didn't pay too much attention to, since this rotation was part of a program and I'm not that into oncology. You may want to study up some on the scans before you go, like how they work and how to read them, plus why is one used instead of another.
I found that there was a lot of patient consulting, and I would think ped onco would involve the usual overbearing parents. The guy I followed didn't wear a labcoat even, just business casual, so that was a little odd. I hardly spoke with him actually and just tagged along here and there, and my disinterest must have been obvious, doh--don't let that happen of course. One thing about onco and especially ped onco--aren't the prognoses often rather depressing, esp for lil kids? How much realistic hope do they have?