when my young children are using facetime talking to their mother they always inadvertently hang up on her. is there not an app that will allow me to lock the screen to preclude them from hanging up?

when my young children are using facetime talking to their mother they always inadvertently hang up on her. is there not an app that will allow me to lock the screen to preclude them from hanging up? or is there another way within the settings parameters to accomplish the same. basically, i just want to be able to talk to someone, put my phone in my pocket and not inadvertently hang up on them.