From an EM perspective, peds abdominal pain is frequently a diagnostic morass... not only is imaging sometimes difficult to obtain/prioritize/radiate, but also has a large component of veterinary medicine as young patients cannot adequately describe their symptoms (esp if they are pre-speech). So, depending on the amount of time you'll have and desired breadth of topic, you could discuss:
1) life threatening abdominal pain - signs, symptoms, workup
2) review of imaging concerns in pediatric abdominal pain - us v. ct v. MRI v. plain films
If you want to focus on endocrine, go with the bread & butter:
1) new onset DKA v. HONC
HemOnc is quite broad when it comes to peds, and often quite specific as well; as a student, I'd avoid this topic.
Hope this helps!
-t