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Anyone ever use a 48 SIM in a GPS watch - the one's suitable for kids?
GPS doesn't require a SIM.
It's a service provided directly by the relevant satellite networks (e.g. Galileo in Europe).
It does not depend on the carrier.
Apps that use the GPS data may required a data connection though - e.g. Maps etc.
What make and model of watch are you thinking of and what services does it provide and use (e.g. calls/texts/data/GPS etc.?
Either way a SIM is a SIM so it, in general, any carrier's SIM should work in any device that uses one.
Hope this helps.