To the best of my knowledge, that's not possible. My daughter got HDs for her two kids, and the same thing happened. I searched these boards and asked for help with set up, but the answer seems to be that since all the Nooks are on one account, all profiles will be on all devices. The only alternative I can see is to create B&N accounts for each child, whether in their name or yours, and then tie the device to it, but I don't know how much control you lose by doing that or if you can tie a credit card in your name to an account in another name. I would not recommend it.
I'm having this same problem is very annoying. I erased it from one tablet and it erased it from the other one.