At bog all the seats are near readers, so they can read you band from there. The amount of passive readers that would be needed to cover the entire park would be in the thousands. Not very cost effective. Even if a cm could read a band the information they have access to is limited. A shop cm can charge to your card but not check tickets, an attractions cm can check your fp but not your charges etc. there is possibly some high level security people that can access a persons entire record but in the case of a lost child, the amount of time it would take to contact the correct person and get them to the child or vice versa far exceeds the the amount of time it would take for a cm to see a tattoo or other visible phone number, and call it.
So no Disney has not claimed they can track a lost child via a magic band.