I second all that. I've never heard of the leg in another's beak trick (I'll let the foot-in-mouth jokes go for the moment,) but its pretty similar to what we did with one nippy basset hound by putting his own ear in his mouth whenever he got nippy, that took care of it quick.
As tex said ATTENTION is probably the biggest reinforcer, so when they bite, or do nearly anything inappropriate just drop 'em and turn away. Technically speaking that is a punishment (by definition a punishment is anything that reduces a behavior, in this case its "negative punishment" ie reducing the behavior by taking something such as attention away).
Another good contingency to implement is differential reward: get them to do a behavior that is incompatible with biting to get what they want. This is more or less two steps, first figure out if there is something maintaining the biting, the old cliche to every trainer is "Every time he bites me I yell at him"; well no $#|T maybe he likes the attention of being yelled at. It doesn't sound like this is the case here, but people are often surprised by what outcomes can actually REINFORCE a bad behavior - so to emphasize something tex said "no drama based reward, no accidental reinforcement." Second, WHATEVER it is that might be reinforcing the behavior, provide a NEW appropriate way of getting that reinforcer. For example when Auggie wants to come out of his cage now he rings a bell rather than screams, because he has learned that bell ringing gets him positive attention and gets him let out of the cage, screaming does neither.
As I wrote this last paragraph I realized it likely doesn't apply to these young birds, but its useful information that I think is important for pet owners to read. Differential reinforcement as described above is USUALLY the best way to change a "bad" behavior. In young birds such as yours the problem is that the tactile stimulus itself of biting is likely the reward for the behavior of biting. This makes if very difficult to "differentially reinforce" a different behavior to give them that same tactile stimulus. However a slight modification will do the trick: redirection, get them to bite something ELSE.
You can teach them that it is not only OK to bite a certain toy or two, but you can praise and encourage them to get their biting out on the toy. This should only be done in conjunction with discouraging biting you (and other people) as described by tex. This way they can get their tactile stimulus reward AND praise when they bite the toy while when they bite you they get absolutely nothing.
I'd like to qualify the redirection strategy by noting that I have broad experience with many animals, however most of it is not with birds and certainly not with Macs. I have no reason to believe that redirection would not be a great strategy here, however I will not be troubled if I am overruled by someone with more mac experience. Some people occasionally claim that if you encourage "aggression" against the toy the pet will become aggressive towards everything. This is certainly not true of dogs or most other animals (the myth that playing tug-o-war makes a dog dominant is just that: a myth) however parrots are unique in many ways.
