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Possible solution:
  • Old Android phone with Wi-Fi only
  • Parental control app of some sort, to only allow access to the wallet app
  • A lighting wallet app, like BlueWallet/Muun
Kids can see their balance, receive payments from you and others, and make payments.
Maybe customize the Android with a piggybank wallpaper, Bitcoin case, and single app on the home screen -- give it a "dedicated piggybank device" feel.