Have you considered purchasing one of those little bells that are often used on desks to call for service? You can train your Yorkie to tap the top of the bell to sound it and keep it somewhere on the floor. The bell could be attached for the floor, to the side of the wall, to the door itself - since it has a flat base it may be easy to do this and your bigger baby won't end up eating it.
Just a suggestion! Good luck!
BELL -->
http://www.turners-retreat.co.uk/aca...llFinished.jpg