In Winforms if the MDI child window is maximized it doesn't receive ResizeBegin (or ResizeEnd) events. It does receive Resize events - why the distinction? If the child isn't maximized it does get ResizeBegin/End events. Is there a nice way around this? There are plenty of ugly ways: calling directly from the MDI container ResizeBegin event to the child for example.
The ResizeBegin/End events are generated when the user starts and stops resizing a window. Implemented by a modal loop inside Windows itself, it keeps the window edge following the mouse cursor when the user moves it. ResizeBegin when he clicks a window edge, ResizeEnd when he releases the mouse button.
Clearly no user is involved when you change the Size or ClientSize property of an MDI child window in your code. So no Begin or End, just the Resize event. And just one Resize event trigger, there's no constant train of them like there will be when the user uses the mouse to resize. Which otherwise explains why Begin/End is important, if you do a lot of work in your Resize event handler then you'll bog down the UI pretty heavily. Common with automatic layout, the visible artifacts are not pretty.
If you really have to then you can simply generate the event yourself. Call OnResizeBegin() before, OnResizeEnd() after you change the window's Client/Size property value. That code needs to live inside the window you resize to get the correct event triggered. Pretty unlikely you should be doing this btw. Do beware that MDI automatically resizes an maximized MDI child window, it of course cannot be maximized anymore when you activate another one. You can't wrap that with OnResizeBegin/End() calls.