Whatever comes within a certain radius of the nest gets dived upon. It's a rule. You may want to watch the bird and discover where the nest is, then take pains to avoid it. Let your fair-haired boy know that he should not play there. I should also point out that your Lizzie has likely deceived you, as well.
Cats are predators, without exception.
Owners would love to believe otherwise, and one such owner, to prove that his cat was special in just the manner you believe your Lizzie to be, began taking special note of her activities. What he discovered horrified him. Cats do not always want you to know what they have killed. My neighbor's cat is the source of a constant parade of mangled chipmunks, birds, and mice on the doorstep, which now reeks of death. Even when cats play with their prey, as you describe Lizzie doing with the baby bird, they are studying it before devouring it.