Setting up this thing blows, after 3 hours of troubleshooting and contacting customer support with no success I finally figured it out.
Before you think about setting up this device you need to set up Google accounts for your kids, they must be under 13 and you need to set up Google family and their family app called family link.
The next thing you need to do is have a device you can set up a second user on. I used my phone and set up another user so I could log my child account into it. I logged into their google account and set family link and then created their Fitbit account. I was prompted to set up their account with my main Google account and then it was really easy after that.
I'm really annoyed I need two user accounts on my phone to set it up. It might be easier to factory reset an old phone and only log in with your child's google account, I'm also not sure why none of this is in the instructions. I was seriously doubting my ability to read for a while there. Also I'm not sure why customer service couldn't explain this to me either.
Anyways good luck to anyone still trying to set it up, it's not technically impossible but it sure feels that way.
I checked the rules and hopefully this doesn't break any of them. I tried checking here yesterday but didn't see any solutions.
