I think your method would work best (just don't like chihuahuas, personally)
I digress, to answer your concern, it's actually pretty simple. The answer can be found in your owners manual, but if you don't have it, all you have to do is lock the car, unlock it, and lock it again in quick succession, and the motion sensor will be disabled in case you leave anyone (or a pet) in the car and don't want the alarm to go off. You'll be able to tell because you won't see the IR sensor doing its usual flashing. Give it a try.
