Connect to my grandchildren

I purchased a Garmin watch for my mother, my oldest son and my 4 nephews because my youngest son really want to have challenges with our family. I can not figure out how to connect my moms (grandma’s) Garmin to the kids. She has an Apple Watch so I probably should’ve skipped the Garmin but I thought a Garmin was required to connect - yet now I am still having problems. Please advise. How do I connect an adults watch to our family teams when the adult doesn’t have children in vivo jr themselves?