You need to set them up first. If you purchased them on the account you want them on, they may be pre-registered in a way to make set up easier, but, even pre-registered Kindles and Fires that show up in Devices before set up, are not completely registered until set up is complete by the user. If the purchase was marked as a gift, they aren't even pre-registered. I'm not sure if pre-registered Echos are supposed to show up in Devices, the same way a pre-registered Kindle or Fire does.
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I'm being extremely clever up here and there's no one standing around looking impressed. What's the point in having you all? - Dr Who