Hide i_children and pp_children away

Hide i_children and pp_children away

They can only be read safely under the Big Structure Lock in
src/misc/objects.c, so it makes no sense for them to be public.

By the way, making i_children volatile wouldn't magically solve
thread-safety issues. The only correct use of volatile is in dealing
with asynchronous changes _within_ the same thread, such as signal
handling.

P.S.: I wish modules were not objects, and I wonder why they are
(they don't don't use threads, nor plugins, nor variables)