J.W.
My kids never had trouble using my cell phone, even when I didn't want them too. :(
Yes I have a smart phone so there is a specific button you have to hit twice. I think they have been able to open the phone long before they knew how to use it.
I would be more concerned if they could find the darn thing if they needed it.
Oh another idea is if you have an old inactive cell phone keep it plugged in as the "emergency" phone. Even an inactive cell, without the sim card even, will place a 911 call.