Heavens, he could be looking at anything! And, no, it's not the 2021 equivalent of us Grans reading a book under the covers! Visual matter is quite, quite different. (Unless your 11 year old GS has a very advanced and graphic imagination!) When my grandchildren are in my house either staying the night, or for a meal the no argument rule is mobiles are left on the hall table. I know that when I say, 'Leave your phone on the hall table' they hear 'Leave your severed arm on the hall table' but that's the rule. And, as they all appear to love coming here, I can't see the problem. As for telling your daughter, why not? Someone has to be the grown up these days.