Here's one approach you can use:
1) Create your individual forms as normal, except use a TLayout, call it "LayoutMain", (aligned alClient) as the parent of all controls on that form.
2) On your "master form", when you want to bring a form instance in to behave as it would as an MDI interface, create a TLayout (call it "FormContainer" or something), place as a child of that (aligned to top) another TLayout (call it "FormHeader"), containing the individual controls for the Form Caption, the Minimize/Maximize/Close buttons and anything else you'd like on the "frame" heading. This is called a Composite.
3) Create an instance of your child form (but don't show it), then parent that instance's primary TLayout "LayoutMain" to your main form's TLayout "FormContainer".
4) On the TLayout previously mentioned (with the name "FormHeader"), give it OnMouseDown
and OnMouseMove
events to provide the ability to drag it around the Master Form.
When you want to maximize the child form inside the master form, you'd just set the outer TLayout "FormContainer" align property to alClient, remembering to store its original Top, Left, Width and Height values within the the form's instance so you can recall them when pressing the Restore button.
True, this is a fairly involved solution, but to the very best of my knowledge this is the only way you're going to achieve what you're trying to do with FireMonkey as it exists today (circa Update 2).
I wouldn't expect Embarcadero to provide any form of MDI emulation as part of FireMonkey "out of the box", as MDI is considered an old-fashioned approach.
The more modern solution would be to use Docking, as the RAD Studio IDE itself does. This provides the very best of both worlds, giving the user the freedom to choose what child forms they want to link into the master form, or display outside of that on their own (or any combination of docked arrangements).