Currently the Fitbit iphone ios app does NOT allow you to change the name of the device, When you have 2 kids each with their own Ace 2 device, and you set them up under Parent/Kids function on the app, they both show up in Bluetooth directory as "Ace 2". Should be able to rename them (eg. "Ace 2 Kid 1" and "Ace 2 Kid 2").
Moderator Edit: Clarified subject