Measure each of your style of keychain and create a rectangle or circle in the same size.

This will give you an idea of what you need to conform to.  Be careful of getting to detailed or the small size will make it hard to see/read.

I've included 5 example designs to start with and to give you an idea of what you can create.

I can't say it enough, jigs are your friends.  When you are batch creating a jig makes it fast to swap out parts without having to reframe every time.

If you don't have a jig or you are just setting up for the first time put your keychains in place and use the Framing Outline option to set your placements of the designs.

There isn't a “keychain” preset but I have found that Wooden Coaster works great.  Uniting your design will usually make it faster to engrave.

Once your designs are complete you may be wondering if you should seal them?

Personally, I don't.  The keychains are already pretty smooth like they might have a conditioner already applied so I don't think that it needs it.  

If you would prefer to though I think that a light coat of a clear wipe on poly wouldn't hurt.  In that case, I would hang them while drying so you can do both sides at once.