I have an Apple Watch, set up with my iPhone. I don't have personal experience with setting up Apple Watch for family members, but can I read between the lines.
Reading Set up Apple Watch for a family member - Apple Support there is a section headed Availability of features and apps which goes thus:
Health and fitness features:
- High and low heart rate notifications are available for users aged 13 and older.
- Heart Rate Variability (HRV) is available for users aged 18 and older.
- Fall Detection is available for users aged 18 and older.
- Users under the age of 13 can use move minutes rather than active calories for their Move goal in the Activity app.
Apple Pay features:
- Apple Cash Family is available to users under the age of 18, allowing them to make purchases and send and receive money in Messages with Apple Pay (United States only).
- Apple Pay using a credit or debit card in the Wallet app is not available with Family Setup.
The following features and apps are not available: medications, respiratory rate, irregular heart rhythm notifications, ECG, AF History, Cycle Tracking, Sleep, Wrist Temperature, Blood Oxygen, Walking Steadiness, Audiobooks, Remote, News and Shortcuts.
Which confirms that the ECG feature is not available when the Apple Watch has been prepared with Family Setup. As far as I am aware pairing with an iPhone where both watch and iPhone will use the same Apple ID is the only other way of setting up an Apple Watch. Hence my use of presumably. Examining the limitations in more detail I can see that these are features that often require interaction on the phone, which you wouldn't have with a watch prepared via Family Setup.
As noted previously this is a user to user support forum. While there are some community specialists who will sometimes respond to threads, in the main they are not going to try to reinterpret the meaning of Apple's support documents. You can contact Official Apple Support if you'd like to try to get a more definitive answer, but I think it is pretty clear; the ECG and other listed apps/features don't work on a Family Setup watch.
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