Open the GDSII file in edit mode (Set_Load_Memory On), then Open_Structure, 
then use the command Save_GDSII_Structure to save the specified open structure to a GDSII file.  
e.g.

1. Open the file in edit mode (use more memory and will create a different
  and bigger load memory map file with .dbload extension).


  Set_Load_Memory
  On

  Open_GDSII
  /tmp/demo.gds

  Open_Structure
  A_CHILD_CELL

  Save_GDSII_Structure
  /tmp/outputchild.gds

2. Keep opening the GDSII file the way you are doing it now and do something
   different ONLY when you want to save to an output file.

   Open the file in NON edit mode (normal mode, use less memory, what you are
   doing right now).  Then when you want to save, change to edit mode, reload,
   and save.

  Open_GDSII
  /tmp/demo.gds

  Open_Structure
  A_CHILD_CELL

  Set_Load_Memory
  On

  Set_Use_Memory_Maps
  Off

  Set_Create_Memory_Maps
  Off

  Reload_GDSII

  Save_GDSII_Structure
  /tmp/outputchild.gds

  Set_Load_Memory
  Off

  Set_Use_Memory_Maps
  On

  Set_Create_Memory_Maps
  On

  Note that Reload_GDSII might take a while if .dbload memory map is not
  available and if you want to generate .dbload memory map.  If .dbload
  memory map is not necessary, set memory maps to off before reload.

  Remember to reset the edit mode (Set_Load_Memory).