In general, file descriptors opened in one process are inherited by child processes ...
... but the MEMORY space in one process is separate and distinct from the memory space of any other process - even child processes.
So no, a GTK+ widget in a parent process cannot be directly manipulated by any of its children.
Two possible solutions:
1. Set up a communications channel between the parent and child.
The GTK+ "selections" mechanism might be a good way to do this:
Alternatively, you could use any standard Unix IPC, from pipes through to message queues:
2. Use threads in a single process (instead of forking off child processes)