Because I registered them under my Amazon account all my books on my Kindle are archived to their Kindles.
So you are worried they will go to the archived books and download your books. You could deregister their Kindles after you download the books you want them to have and register them again when you want to get them books again. It might be easier to register theirs on a new account just for them. You could then put a prepaid via credit card on their account with a low balance so that they can't buy books with it. When you want to give them books just apply an Amazon gift card on the account.