You might want to check out the
neonatology/PICU/cardiology subforum. We have discussions like this one over there which you may find helpful.
To quickly answer your questions:
PICU is a 3 year fellowship completed after a 3 year peds residency.
A peds intensivist treats people from birth to 21 years, but not premie babies, or babies who haven't been discharged from the hospital yet. That's left to the neonatology.
Case type depends on the institution and resources there. Basically these are the kids on ventilators, kids with severe sepsis, severe DKA. Generally the sickest kids in the hospital. Intensivists also handle ECMO, post surgical patients (similar to what you'd see in an adult SICU), bone marrow and solid organ transplants.
The 'newer' part of PICU is taking care of heart kids, though sometimes this is done by pediatric cardiologists or double boarded PICU/cards. It mainly entails the congenital heart defects as they go through various corrections and support.
So it's a wide range of patient age, pathology, medicine, sedation/anesthesia and surgery.
