Leave the chick with them and try not to disturb them at all. This means no looking in the nest for a few days!
Give the parents lots of soft foods......thawed green peas, corn, mashed boiled egg, sprouted seeds, shredded zucchini, etc.
Often finches don't feed until the second chick hatches, especially if it's their first clutch. Plus, each chick will slowly absorb the yolk sac (yellow patch on tummy) and once this is absorbed is when they will show a feeding response.
BTW, six eggs isn't unusual for Zebras.