Win32 - Child Window Painting - c++

Question

I have a main window that contains two child windows (Child1 & Child2). Moving Child2 on top of Child1 and then moving Child1 away, the Child 2 client area and non-client area gets painted onto Child 1.
See image here: child windows
Any help please?
#include <windows.h>
LRESULT CALLBACK WndProc(HWND, UINT, WPARAM, LPARAM);
LRESULT CALLBACK WndChildProc1(HWND, UINT, WPARAM, LPARAM);
LRESULT CALLBACK WndChildProc2(HWND, UINT, WPARAM, LPARAM);
int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR szCmdLine, int nCmdShow)
{
WNDCLASS wndclass;
HWND hwndMain, hwndChild1, hwndChild2;
MSG msg;
//register main window class
wndclass.style = CS_HREDRAW | CS_VREDRAW;
wndclass.lpfnWndProc = WndProc;
wndclass.cbClsExtra = 0;
wndclass.cbWndExtra = 0;
wndclass.hIcon = 0;
wndclass.hCursor = LoadCursor(0, IDC_ARROW);
wndclass.hInstance = hInstance;
wndclass.hbrBackground = (HBRUSH) COLOR_WINDOW;
wndclass.lpszMenuName = 0;
wndclass.lpszClassName = TEXT("MainWindow");
if(!RegisterClass(&wndclass)) return 0;
//register child window class
wndclass.lpfnWndProc = WndChildProc1;
wndclass.lpszClassName = TEXT("ChildWindow1");
if(!RegisterClass(&wndclass)) return 0;
wndclass.lpfnWndProc = WndChildProc2;
wndclass.lpszClassName = TEXT("ChildWindow2");
if(!RegisterClass(&wndclass)) return 0;
//create main window
hwndMain = CreateWindow(TEXT("MainWindow"), TEXT("Main Window"),
WS_OVERLAPPEDWINDOW,
CW_USEDEFAULT, CW_USEDEFAULT,
CW_USEDEFAULT, CW_USEDEFAULT,
0, 0, hInstance, 0);
if(!hwndMain) return 0;
//create child windows
hwndChild1 = CreateWindow(TEXT("ChildWindow1"), TEXT("Child Window 1"),
WS_CHILD | WS_CAPTION | WS_SYSMENU | WS_VISIBLE,
10, 10, 200, 300,
hwndMain, 0, hInstance, 0);
hwndChild2 = CreateWindow(TEXT("ChildWindow2"), TEXT("Child Window 2"),
WS_CHILD | WS_CAPTION | WS_SYSMENU | WS_VISIBLE,
250, 10, 200, 300,
hwndMain, 0, hInstance, 0);
ShowWindow(hwndMain, SW_SHOW);
UpdateWindow(hwndMain);
while(GetMessage(&msg, 0, 0, 0))
{
TranslateMessage(&msg);
DispatchMessage(&msg);
}
return msg.wParam;
}
LRESULT CALLBACK WndProc(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam)
{
switch(msg)
{
case WM_DESTROY:
PostQuitMessage(0);
return 0;
}
return DefWindowProc(hwnd, msg, wParam, lParam);
}
LRESULT CALLBACK WndChildProc1(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam)
{
return DefWindowProc(hwnd, msg, wParam, lParam);
}
LRESULT CALLBACK WndChildProc2(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam)
{
return DefWindowProc(hwnd, msg, wParam, lParam);
}
Thanks.