I'm trying to achieve this here:
where if you hover over the border between the list view and the tab panel you get to increase the size of the top window whilst decreasing the size of the bottom window and vice versa but I'm at a total loss as to what flags need to be set to enable child window moving by the user.
It is not as easy as setting child window flags. The narrow stripe between the upper and lower child windows has to be a background window that implements a splitter window effect by handling mouse messages on the stripe. This link outlines the basics: