Having a website design and hosting business for a few years now, and losing my butt in it, I have begun to try to figure out how to turn over the keys to my clients.
How does one give the domain name info such as login and pw to a single client to renew their domain name, when more than one are registered at the registrar?

I would give them the information they need to register the domain in their own names, along with the information on how to TRANSFER their domain from you to their registrar of choice - even if it's the same one you're using now.

A very simple process if they create an account where the name is registered. Most real registrars offer a free push to the new account. Some of the fly by nighters do not.