Hello, Nicolene -
To clarify what you are asking about . . .
The SIM card you have for a Polaroid GPS Kid Tracker Smart Wristwatch has been registered by a previous owner? Or, do you mean you registered the SIM card and it does not work with the watch you have?
Just in case . . . this is the link to the Installation Guide for the Polaroid GPS Kid Tracker Smart Wristwatch:https://kidssmartgpswatch.com/pages/faq
If the SIM card you are trying to use in the watch was registered previously by someone else, then, you either need to find out how to change that registration or you need to register a new, different SIM card.
I suggest you contact the manufacturer and ask your questions:
By the way, your question landed in the Fixya category for Polaroid Cell Phones.