To start the child the parent would either
call exec() to start the child running a different program, or call a
new function that starts the child with a parent-provided entry point
and whatever memory mappings the parent set up.
This is what I plan to do on the OS I'm writing
(manipulating the child's state won't require any additional
primitives beyond regular file I/O since literally all process state
will have a file-based interface).
In that case you don't need *any* primitive except create_empty_process(): you can do exec() by opening the file, writing to /proc/<child>/mem and then to <proc/<child>/pc-and-go.