Husband and I are wanting to buy an apple watch for his daughter and use her apple ID that is linked to our family sharing account, my question is: Her Mum has gotten her an iPhone that is using an apple id linked to HER family share, is there a way that she can sync her iPhone to the Apple watch with separate apple IDs?
Set up your Apple Watch - "Learn how to set up your new Apple Watch with your iPhone."
https://support.apple.com/HT204505
The Watch will be paried with the Apple ID that is on the iPhone to which it is paired. Change the Apple ID on the iPhone so it is the daughter's and then the daughter can pair the Watch and iPhone to the same ID.
Set up your Apple Watch - "Learn how to set up your new Apple Watch with your iPhone."
https://support.apple.com/HT204505
The Watch will be paried with the Apple ID that is on the iPhone to which it is paired. Change the Apple ID on the iPhone so it is the daughter's and then the daughter can pair the Watch and iPhone to the same ID.
iPhone Pairing
