MDI is the term you're looking for: Multiple Document Interface. You set the IsMdiContainer property of the parent for to true in the designer and then you assign that parent form to the MdiParent property of each child form before calling Show.
Note that any controls that you have already added to the parent form will float over the MDI client area, which may not look ideal. If not, you can add a Panel to the form and set its Dock property to the appropriate edge, then move the existing child controls into that.