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Or the parent should just take the phone away! If the kid won't listen to it quietly then they should do that thing that I believe is called "parenting". Bring a picture book or something for them if they need to be entertained without the phone.

This was done by my parents when I was a young kid. I wouldn't turn the volume down on my Game Boy on a flight, so my parents took it away from me until I promised to keep the volume down, which I did after that.

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I've been on a flight where a set of parents took away their child's tablet, not for being noisy but as punishment for some other bad behaviour. What resulted was 6 hours of a child screaming on an 8 hour flight. Aside from wanting to punt the little shit out the door, I was almost impressed at the kid not giving up after a few minutes, and then hours when nothing changed.
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