Proper way to declare ReferenceCountedObjectPtr member in header

I'm pretty confused by this.

Given MyParent.h:

class MyParent {
    ReferenceCountedObjectPtr<MyChild> myChild;

Can I use a forward declaration for MyChild?:

Or should I always need to include MyChild.h?

Most times it has worked the forward but sometimes it would complain within ReferenceCountedObject that MyChild is incomplete and now that I've updated to the latest git I'm getting this issue in many places where I never used to.