How to allow my Grandson’s parents who use Android to become members of ‘Family’ so they can use ‘Find My’ to be able to locate where their son is from his stand alone Apple Watch.

As Above.

As an IPhone user I set up ‘Family’ so my grandson can have access to us via a stand alone Apple Watch without a mobile. I have control over his watch so he can call myself and his parents and Vice Versa. The Family group is working fine. I now want to add his 2 parents who use Android mobiles. Invitations have been sent but what next.

How can they get to use ‘Find My’ so they can track him as I am able to do?

thanks