some ideas-

what about a baby cam type of thing?
so they can check /watch her on the TV?
http://www.x10.com/key_products/baby-cam.htm
or
http://www.agis.com/Eldercare-Basics...-Call-Systems/

or even a motion sensor of some sort so she just waves her hands and it will turn a light on or make a buzzer sound??

a whistle?

or old fashioned ringer type or pull string bell?