While I agree with Robbie that Daisy's behavior is understandable, I agree with you, Amy, that you need to deal with this ASAP. You will want to do this in a positive manner.
This is what positive trainers recommend. The boys need to become miniature treat dispensing machines. Let them carry very small treats (or pieces of kibble) in their pockets. When they come in, Amy gets treated. Period. If need be, they can start by just dropping it on the floor, very gradually advancing to holding it out for her and then petting her while giving it to her.
If you are serious about helping her past this, I'd let the boys control other resources as well. Let them feed her. (She's old enough that she shouldn't need free-feeding). Let the boys introduce new toys. Encourage the boys to play her favorite games with her. She needs to have lots and lots of positive associations with the boys coming into the room.
Another thing that positive trainers say is do NOT punish growling. Say "NO" but don't punish. Teach a dog that he's going to be punished for growling and he may well stop. Then, what have you got? A dog who bites with no warning. Better to have a growl first so everyone knows when Amy is feeling threatened.
Please note that I'm recommending that you ignore the growling. I'm saying deal with the growling per the plan above. That's not ignoring it.
Does this make sense?
