If this isn't a fantasy craft project that she makes up for fun, but something she actually has to design and execute using scientific prinicpals, this sounds like an electronics project to me.
I'd get a science kit that utilizes simple electrical circuits. Snap Circuits is a popular brand. http://www.snapcircuits.net/products/product_details/snap...
There are also plenty of simple sensor kits on the market. They come packaged as room/burglar alarm. If you have a Hobby Lobby or Hobby Hub in your area, you can find such things...or at the very least the materials you'll need.
Otherwise check out these sites:
www.discoverthis.com
www.fatbraintoys.com
www.homesciencetools.com
Or these books for project ideas and how to do them:
http://www.amazon.com/Make-Electronics-Discovery-Charles-...
http://www.amazon.com/Electronic-Circuits-Projects-Engine...
By the way, there are a couple of real products out there that do this. There are actual pet GPS lojacks (like the kind you put in a car) that the pet wears on their collar. It sends out a signal so you can find your lost pet using your cell, computer, or with the help of a rescue service, and there are actual room sensors you can buy that can sense when a pet is getting into the trash or climbing onto countertops and furniture and will sound an alarm. Perhaps she can contact the manufacturers and get the science behind how the things work and do a low-tech modified version of her own.
See:
http://www.spotlightgps.com/spotlight-gps-pet-locator.aspx
http://www.drsfostersmith.com/product/prod_display.cfm?c=...