Experimenting with Screen Time I enabled a passcode on my personal iPad after I’d enabled “Share Across Devices” . I then found I could disable it without reentering the passcode. Which kind of defeats the purpose of a parental control passcode.
Then I turned it on again, and this time I was asked something like: “Is this iPad for you or your child?” . Once I chose child I could no longer remove the passcode without entering it.
“Share Across Devices” then turned itself off.
When I turned “Share Across Devices” back on then I had to reenter my Screen Time Passcode. After than Screen Time Passcode was disabled.
Maybe this isn’t exactly a bug, but it certainly is awkward. I wonder if “Share Across Devices” uses iCloud ID or iTunes/Store ID.
Screen Time for family is enabled through the “Family Sharing” screen.
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 I think Share Across Devices Requires Apple’s two-factor authentication, which seems to rely on SIM-hack-friendly justly scorned phone number authentication. Yay Apple.
 Remember when iOS was going to allow multiple accounts on a single iPad? Android did that for their now defunct tablets.