I would knock all this off and just feed the chick starter and fresh plain water to all.But my wife cooks them dinner every night from combinations of eggs, oats,apples,tomatoes,basil,and other things.
And give the older bird some calcium as @Wyorp Rock suggests.
This is what I do when I have a bird that is acting off:
I isolate bird in a wire cage within the coop for a day or two....so I can closely monitor:
-their intake of food and water,
-crop function(checking at night and in morning before providing more feed),
-and their poops.
Feel their abdomen, from below vent to between legs, for squishy or hard swelling.
Check for external parasites or any other abnormalities.
Best to put crate right in coop or run so bird is still 'with' the flock.
I like to use a fold-able wire dog crate (24"L x 18"W x 21"H) with smaller mesh(1x2) on bottom of crate under tray.
Then you can put tray underneath crate to better observe droppings without it being stepped in. If smaller mesh is carefully installed, tray can still be used inside crate.