DS (11) has an iPod touch which is linked to my account. I did this because I thought if I set him up with his own account, I would have to put in my credit card details and there would e the risk that he might "accidentally" but stuff without asking me. As it is he asks, and either I say yes or he gives me the money from pocket money and I buy the app.
However, this means that my own iTunes account is clogged with millions of crappy apps that I don't want on my iPhone and keep having to manually remove.
DD wants a Kindle for Christmas and it seems the same thing will happen: she will end up with all my books on her Kindle as well as her own and vice versa.
Dd1 has a iPod touch and ds1 has a kindle. They have their own accounts for these which are only credited when they get vouchers. It is not necessary to link a credit card to them as far as I'm aware. In fact, thinking about it my mother has this arrangement with her iPad too as she is frightened of downloading stuff and being charged for it!
I am not sure about iPods, but you can have several Kindles on the same account, but not the same books on all of them. You give each Kindle a different name and then define which one the next book purchase is to go to.
I did NOT want the entire Harry Potter series on my Kindle... .
12yo dd has a kindle attached to my account (I have one too). It's quite simple, when I buy the ebooks I can click on "my kindle" or "dd's kindle" so my books come to mine, and all the teen stuff goes to hers. I get to choose the books (so the poor child gets loaded up with all sorts of free worthy classics as well as the books she chooses). DD is quite happy with this as in general I don't mind buying the books she wants.
YOu can also go onto the Amazon webpage and Manage Your Account, and delete or download books to each kindle. So it's quite easy to keep your books separate.