without manually setting them, my aspect ratio will be hard to estimate
As long as the backbuffer width/height is larger than the child window's client width/height you'll be fine. Just don't render to the entire backbuffer.
They are a couple approaches but the simplest would be:
Get the child's client rect.
Set the viewport to the child's rect size. // sets the portion of the backbuffer to render to.
Set the view to whatever you want.
Set the projection with the aspect ratio of the child. aspect = (float)cRect.width/(float)cRect.height.
Draw the scene
device->Present( &cRect, &cRect, hChildWnd, 0 ); // present the cRect area of the backbuffer to the cRect area of the child window
EDIT: Yes, setting the viewport properly is the key.