After 3 days of obsessing over this topic, I've found a solution/work around. Hoping this helps people in the same issue.
Goal: To be able to manage my kids Fitbit Ace 3 trackers on my own phone.
Obstacles:
1. Google will not let you set up a Ace 3 tracker without a child account.
2. Google will not let you have a child account on your own phone because for some reason Google wants your child to have their own device.
3. Only child accounts created WITH Family Link will work. If its an existing supervised child google account - it wont work as of today.
My solution:
1. Create child account under family link app. Only under the Family Link app.
2. Create extra users on your Android phone. On the Pixel 8 phone, you Allow multiple Users under settings. Then create a user account. In my case, I created 2 accounts.
3. Login using the child account you created in Family link. This will prompt you to authenticate that you are the supervisor - so you'll need another device to do 2 factor authentication. I had my old phone that I am still logged into, so i was able to verify there.
4. Install Fitbit and login, set up the Ace3.
It worked for me, hoping it works for others here.
