How to lock volume on iPad for Autism support?

My son has autism & wont wear headphones. He loves YouTube because it had been easiest to operate. But… we can’t keep him from turning volume up as high as it will go

we’ve tried guided access and everything we’ve found online and in forums.

how to we keep the volume at a fixed spot in all apps? Is there more than one way? Give me all the suggestions.

again no headphones but we just can’t fix it. We’ve tried. It’s a 9th generation updated.

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