If an adult comes to tell you that they are being regularly harassed and beaten up by a coworker, will they be told to be careful about retaliation, and oh shucks since there's no direct proof of anything we can't separate you from your attacker?

No.

And I think there's a good reason (not that I particularly agree with it). People, the justice system in general, trust adults to be reasonable and act within the law so anyone who goes to a police officer and says X man attacked me is believed. With children, there's always the question in the back of the mind "did they lie to get their enemy in trouble? are they pushing boundaries? Are they just seeking attention? Are they exaggerating?"

For an adult, just your word that you were attacked is often good enough to kick start an investigation for physical evidence. For a child attacked by another child, their word is often not seen as good enough.