Base class have derived class like member

I am creating widget system and I have a big problem.

My base class Widget is form for creating other part of widget system(like Image, button etc.) so I want to inherid from Widget (every part has to have atributes of Widget). But every Widget have to be placed in PanelWidget which is also inhered from Widget (Widget have pointer to PanelWidget like meber variable).

This doesn't compile, because Widget doesn't know how PanelWidget looks like. I already used forward declarating but that didn't helped. I can solve this when I put both classes to one header file, but i dont want to do that(it is messy).

So what should i do?