It all depends on where you choose to work. If you work in an outpatient setting, you would see kids with minor illnesses and injuries (e.g. colds, flu, etc.) ... kids needing physical exams for schools
, camps, etc. ... kids needing immunizations ... kids need following, monitoring, management, of chronic illnesses such as diabetes, GI problems, etc.
If you work in a children's hospital, you could see anything -- depending on the unit you work on. You would find kids with cancer on the oncology unit, etc. There are units that specialize in the care of every kind of patient with every kind of health problem -- kidney disease, heart defects and heart disease, infections, respiratory illnessess such as asthma, GI problems, kids having transplants, etc. There would be kids having surgery, kids requiring intensive care, a unit specializing in the care of newborns, etc. You could see just about anything in a major children's hospital.
What types of patients you see (and their diagnoses) depends on where you choose to work.