To reproduce:

Add an mdi child to a form:

this.IsMdiContainer = true;
new Form
{
    MdiParent = this
}.Show();

ThemeResolutionService.ApplicationThemeName = new VisualStudio2012DarkTheme().ThemeName;

Start the application and maximize the child form. You will see that there is no status bar for the child form. The same behavior occurs with the VisualStudio2012LightTheme.

Workaround:

You need to set images to the buttons in the title bar:

this.FormElement.MdiControlStrip.MinimizeButton.Image = this.FormElement.TitleBar.MinimizeButton.Image;
this.FormElement.MdiControlStrip.MaximizeButton.Image = this.FormElement.TitleBar.MaximizeButton.Image;
this.FormElement.MdiControlStrip.CloseButton.Image = this.FormElement.TitleBar.CloseButton.Image;


this.FormElement.MdiControlStrip.MinimizeButton.ShowBorder = this.FormElement.MdiControlStrip.MaximizeButton.ShowBorder = this.FormElement.MdiControlStrip.CloseButton.ShowBorder = false;