Depending upon how old your neice is, and if she wouldn't make you "prove" it, you can always do the old sneaky trip out to the hen house, put a watered down elmer's glue on the egg and then sprinkle with glitter. If she is young enough, that would send her into spasms of delight. UNTIL she went and told her teacher or someone who wants to know the whole story . . . then its a whole nother' story!!!!
From a teacher's perspective: Write the question, write the prediction, perform the experiment, write a conclusion!
I think it would be a fun experiment
Why not try just a little? For the fun of it and she may learn something too. I very much doubt it would hurt the chickens . I mean they eat all sorts of goofy things and live through it
Proceed with small amount of glitter feed. Then, before neice gets up in AM, very early go out to chicken coop and use glitter glue to coat eggs in nest.