You may have up to 6 Kindles on an account. You may share content between them - the key is ON THE SAME ACCOUNT. That means each Kindle is registered using the SAME credit card. There is an option to list more than one credit card, but you must manually change the card for each purchase. That would be a pain in the neck for me. I would rather have all charges go through my card and then settle up with the others. That's what a friend of mine does. She and her two sisters all have Kindles, on a sister's account. That way when one of them buys a book, each sister has access to it. Once the book is purchased, each Kindle can wirelessly download it and all of them can be reading the same book at the same time. They each keep a list of what they have purchased and send their sister a check. It works fine.
Obviously you would not want just anybody sharing your Kindle account - I believe it would work best for family members or close friends. I would have both of my daughters on my account, and future sons in law, but not anybody else!!
A point to remember is that if you share your account everyone can see every title you have. Not a problem for me because I am reading mainstream literary firction only, but if you choose to get erotica (and there is plenty of it in Kindle format already!) or even just something you'd rather no one else see, you should keep your account private.
There is no way currently to block anything from your child who might be sharing your account. You have only your normal parental oversight and any rules you might make for the use of downloading.
Don't let this dampen your enthusiasm for the Kindle!! It's a wonderful, wonderful machine. Just sharing some practical information gleaned from the
Amazon boards over the last few months.