Family setup Apple Watch Limitations
How can you instal apps on a child’s family setup Apple Watch, and are all the normal apps available?
Apple Watch SE
With Family Setup, your family member who doesn't have their own iPhone can use their Apple Watch to do things like make phone calls, send messages, and share their location with you.
Note that some Apple Watch features depend on having a companion iPhone, and aren't available on an Apple Watch that you pair using Family Setup.
For more info, see the 'Availability of features and apps' section in:
Set up Apple Watch for a family member - Apple Support