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That is rather a puzzler. I need to ask, Is Pokemon Go already present on his Kindle Fire device?
OK. I presume the primary login account on your grandson's Kindle Fire tablet device, is yours?
So when you say 'add it to his part', you mean add it to the apps that he can access when he uses the Kindle Fire, correct?
yes, sorry for the delayed response
How did you buy (or, register for) the use of the app? I am wondering whether having two users access it (you, and your grandson) is beyond what you purchased, maybe?