These two functions aren't that hard to implement run time into Gtk+. If you look at gtkfixed.h (Gtk+ version 2.0 API) you can see that all the children are stored in a GSList. You can iterate this list to find the desired child, take it out of the list, re-insert it where you want (all by using GSList functions) and emit an expose signal to the GtkFixed parent widget.
Stian