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I have no advice, but your post is really interesting.
You might consider supplementing their care by handfeeding, in a
couple of weeks. I hope that the babies are safe from any of the other birds in your aviary.

When I used to have a pair of bourkes, I allowed them to reproduce. They had two clutches, but only two eggs were fertile in the second. I handled the chicks as they grew up, from before they had feathers and could open their eyes. They turned out quite tame actually.

There is a simple solution to this, here it is. If the hen doesn't mind you checking on them dailey, then take some time to handle them. It doesn't have to be a long time maybe a few minutes a bird. I have done this successfully and its worked time and time again. But remember it does depend on the hen.
Now i have to add my own opinion on handfeeding grasskeets. I don't believe in it. This is why: Most species of grasskeets tend to be pretty docile so handleing chicks early or even after they wean works well to tame them down. In addition when we hand feed we are changeing the natural dynamic and know how of the bird in the future. I know of too many people who have gotten a H.f. bird and later down the road wanted to set the bird up for breeding and it didn't work out so well, due to too many variables. I suppose this is enough so I'll step down off the soap box. ( i dont mean to offend its just an opinion )

I have to agree with jeffm about handling the birds early. The process of getting the birds to trust people is the key to having a tame bird.
Examples:
I have a male cockatiel that was parent raised. He was handled all the time by children and is EXTREAMLY tame.
I have a male and female Red-Rump that were hand fed late and are not as trusting of people.

When you are hand feeding the important part is the human interaction that the bird of any species receives. A bird that is comfortable around people and their hands will be a tame bird. It is not just the fact that they received food from a person that makes them tame.

This is my opinion & experience, not a scientifically proven fact