A Nanny is mysteriously directed to the wrong house and is demanded to kill captive children for a soul hungry Entity whose time is running out to earn immortality; she must choose to save the children and escape or be tortured, forever captive with them.

There is no industry wide convention for logline formatting as there is for script formatting for example but loglines are a common tool used in the industry to sum up a story.
Whether you use this tool to structure or pitch the story will determine what type of logline you need.

A pitch logline will likely benefit from less words, 30 and under, and focus on "hooks" this type of logline should be used after a script is finished and ready to be shopped around.

A structuring logline should focus on plot and character as this will road test the concept via the core elements of your story and help you structure the whole plot for the script.

I'm not sure what stage of development this concept is at and unless told otherwise I and most members on this site comment under the assumption it is still in its early structuring stage. If this is the case all previous comments still apply, if not and you are about to pitch the script then perhaps best to re think the logline with a focus on the stakes characters and the inciting incident and less about the genre.

And yes I to find this web site is like a gold mine of feedback...

Great idea recording the logline and playing it back later. That's gold...

There might be a balance to strike here. And possibly a word count limit to test. One way I did my most recent post (face palms because I could have done it sooner) was to read it out loud into my digital recorder and let an hour go by, come back to it and listen to it. If it confused me then it became clear something in that log line came from my imagination (to explain as a log line) versus explaining like a news headline.

Hearing it out loud on a recording exposed the issues. Some were glaring. Other parts of the log line were clear but the sequence of the log line was "off".

So, I had to write it like a before, during, and after news alert.

A news channel is sometimes a great way to learn concise story-telling. And every once in a while, someone captures a story perfectly with just the right amount of description. Plus, it's understandable and it would be possible to say to say what they said as an elevator pitch.

Difficult to so, but so worth it.

*rushes to his library to re-read "Selling Your Story In 60 seconds" again...