Stress
You won't be able to heal everyone, and the cases that are beyond your control can be truly heartbreaking. You'll need to develop a thick hide and be able to deliver bad news to parents—as well as to the children themselves—while remaining strong and at the same time coming across as sympathetic and compassionate.
Just because they're kids doesn't mean that all they get are minor cuts and scrapes. It's pretty much guaranteed that, sooner or later, you'll find yourself dealing with some truly horrific situations; and when you do, your patients and their parents will look to you for comfort and guidance.