The numbers you see in e.g. R2013 are the wanted graphic handles. Before R2014a, which introduced the new graphics library called "HG2", doubles have been used for graphic handles. In consequence your code is correct already:
Children = get(handles.untitle, 'Children');
set(Children(1), 'enable', 'off');
By the way, it is not save to assume, that a specific object is the first children. Recognizing an object by its 'Tag' and/or 'Type' is safer. Even more efficient is to store the handle of the object during the creation already instead of searching it dynamically.
