How can I make an entry (in this case a page from structure) only visible to users with cp access?
Craft has a handy {% requireLogin %} tag; this seems like the quickest and easiest solution for you.

If this particular entry has its own Twig template you can just put this tag at the top of the file.

Alternatively, as you say you could create a lightswitch field, add that to your entry type and simply tick the entries you would like to be password protected.

A simple piece of Twig code that checks the lightswitch and does what you need could look like this:

{% if entry.requireLogin %}
    {% requireLogin %}
{% endif %}