Handle on a sub-FSM (child scenario) spawned from a parent scenario via the spawn_fsm macro of FSL.Machine.

The parent keeps one handle per child in its context appdata, under the :__children__ key, indexed by the local name it assigned the child (as:). The ref is the monitor reference returned by spawn_monitor, used to correlate the {:DOWN, ref, …} safety-net message when a child dies without having sent its {:child_exit, name, …}.