H.W.
I walk over and take it from my son if he's supposed to be off.
He's ten. He knows I'm the boss. I understand that an app might do something which would potentially eliminate an argument, but my son needs to be able to do what I ask him to without a meltdown, or no iPad privileges for a while.
If a child is too young to grasp that concept, then maybe too young for iPad time.
If the child is not with you, maybe no ipad and they can do other things? I've never left caregivers with potentially fraught situations.... and, as a former caregiver, I HATED when parents did that to me. Just something to think about. There are PLENTY of other things for a child to do!