Number of CMDIChildWnd(s) in CMDIFrameWnd

Is the child frame of single type ( i.e. only one class for childframe windows ? ) ?

CMDIFrameWnd has this member:
public:
HWND m_hWndMDIClient; // MDI Client window handle
You could enumerate its child windows, which are your CMDIChildWnds

You could use VladimirF's approach using EnumChildWindows API , in which case beware of this: If a child window has created child windows of its own, EnumChildWindows enumerates those windows as well.
You could use VladimirF's approach using GetWindow with GW_CHILD and GW_NEXT/PREV
You could maintain a static int member for the CMDIChildWnd derived class in your project to represent the count of MDICHildWnds. And increment it in the class' constructor and decrement it in the class' destructor, in which case, beware that this will work only if your app uses one childframe class alone.