i am trying to design UI using wxFormBuilder. I have added wxFrame -> wxGridBagSizer -> wxSplitterWindow.
After this point wxFormBuilder is not allowing me to put any windows under splitter window. I tried putting almost every widget. I also tried putting sizers under splitter window. But nothing is working.
All the widgets go at the same level as spliter window.
In formbuilder, the wxSplitterWindow control accepts 2 (and only 2) children.  Those children can be either wxPanels or wxScrolledWindows.  You can then add a sizer any other controls to those you want to those children.  
If you use panels for the children, make sure to use the wxPanel from the "Containers" page and not the panel from the "Forms" page.
If you want to know which items are allowed to children for a certain control, you can look at the file objtypes.xml in the xml folder of the wxFormbuilder of the application.
wxformbuilder does not allow putting windows under wxsplitterwindow