I’ve brought a tablet on airplanes to watch movies with kids on long flights, but we bring headphones. Flights are the only time we do this.

There is nothing about a tablet or a flight that requires letting them blast audio at full volume. It’s not even a good experience.

That's rather defeatist. Surely you believe there are other options.

We traveled with a single Nexus 7 and one pair of headphones shared by three kids. Having to take turns taught them to be OK with having entertainment, being a spectator, or being bored. And they understood that if we ever heard it, they'd all have to be bored for a while.