It would not be against code to put a shut-off in the garage, in my jurisdiction. I have never heard of, or seen a locking gas shut-off. If you have a ball-valve, can you drill a hole in the end of the handle to some-how lock it to an eyelet screwed into the wall? You cannot lock it in the open position, but you can in the closed position. If it is a ball-valve, you can take the handle off when it is closed.

There is also an electric solenoid valve you could put on. All you need is a switch to turn it on or off.

The handle usually comes off a gas valve with a small screw. Remove the handle and give it to the caretaker. She can take it with her or hide it from your mother. She should be able to use the handle without the screw to turn on the gas.
If the stove has standing pilots turn them off and use a sparker to ignite the burners.

Thanks. I think I may go with Joed's idea. Its very simple, yet effective. I just have to be sure my mother-in-law doesn't know. She always was a worker, and turning it on without the knob is not beyond her capabilities. lol