As per my understanding you want to add child screen from another child screen.
For example -
MDIParentForm > open first child > on first child open 2nd child screen and add this screen to MDIParentForm.
MDIParentForm = Parent container.
Form2 - First child screen
Form3 - Second child screen
Add below code to your second child form from where you want to open and add screen to MDIParent form.
Form3 objFrom3 = new Form3();
objFrom3.MdiParent = Application.OpenForms["MdiForm1"];
objFrom3.Show();
If you need more help then let me know.