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Hello @JessDaus, have a warm welcome to the Fitbit Community, it's great to have you on board. 
I appreciate you have brought this to my attention. Tell me, are you trying to set up a Fitbit Ace for your daughter or another Fitbit device? If you're trying to set up Fitbit device other than the Fitbit Ace, your daughter will be able to sync it with 2 or more mobile devices.
If you're trying to set up a Fitbit Ace, your daughter will only be able to sync it with the mobile device where it was paired. To know more about Family Account and how the Ace works with them, please refer to the following article: How do I set up a Fitbit Ace?
I hope this can be helpful, if there's anything else I can do for you, please feel free to reply.
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