Hello Mister Nice Guy,
Welcome to the forum.
Looking at the photo you have posted, it looks like you have a very young pied white or a chestnut flanked white. Chestnut flanked whites have a dark beak when they hatch and the hens also have dark patches on their heads. It's hard to give you a true colour without seeing photo's of the parents.
Zebra Finches use markings on the inside of the youngsters beaks when rearing, these marks show up in low light, this allows them to feed their young through the night. There could be a number of reason why they stopped feeding the two youngsters. I have found that hens are more susceptible to problems than cock birds.
Zebra Finches learn from others and it may be that your hen has only human habits to pick up from, generally, if a bird is removing the husks from millet they are eating the Kernel. Just a question, what makes you think that your young hen is not eating seed.
I hope this answers some of your questions and if you need any further help, please let me know.
Gary