For linux we have the users and groups concept, the distro can add an adult group and when you give your child a linux a device and create the account you would just chose adulr or minor , or enter a birthdate. No freedom lost for the geeks that install Ubuntu or Arch for themselves and we do not need some extra hardware for the rare cases where a child has access to soem computer and he also can wipe it and install Linux on it. Distro makes can make the live usb default user to be set as not adult. Good enough solutions are easy but I do not understand why Big Tech (Google and Apple) did not work on a standard for this. (maybe both Apple and Google profits would suffer if they did)