Hilchos Lashon Hara chapter 8, section 3

Sometimes there is lashon hara even about a child. For example, saying bad things about an orphan whom people are raising in their home. This could cause them to send him away. The same is true in any case where the story could cause harm or distress to a child.

If his goal in telling is to stop harm caused by the child, and to guide him onto the right path, it is allowed. But he must first know clearly that the story is true, and not rush to rely on what he heard from others.