Screen Paintng

I have a splitter window in my childform in and MDI app. This splitter is made up of a CTreeView derived class on the left hand side and a CView derived class on the right hand side.
When clicking on the tree nodes I'm creating modeless dialogs on the right hand side by making them children of the CView derived class. This all works fine.
The problem I'm having is that if there is more than one dialog open and they overlap they all get screwed up with their painting ad the way they look.
What do I need to do so that the dialogues will stay painted properly when they are moved about and overlap?