Ay, well, from a mother and grandmother point of view, sibling rivalry is not something to encourage but, quite the opposite, to discourage (it's one of those unavoidable things when raising more than one child and believe you me when I tell you it needs no encouragement!).
Different species don't train the same way and even different individuals of the same species would not learn at the same pace or the same way so I am afraid that this is fact of life (quakers are actually much more clever than any psittacula but they are also quite stubborn when they want to). Besides, children will lose interest real soon on anything that requires constant work without immediate results (and that's why I don't recommend parrots for them).
Personally, I would just make it a point of admiring their birds profusely and how far they have come, comment on how much the birds love them and remark on how the birds need their little 'mothers' just like a baby needs a human mommie (little girls respond very well to this kind of thing).





