Be prepared to lose the chick that is moving less.
I often order bantams, along with the larger chicks, they ship in a separate compartment, but the others keep them warm enroute.
But, the smaller chicks are every bit as active as the larger ones, often more so, more hyper.
It does make sense that smaller chicks would eat and drink less.
Are they keeping the small one away from the heat?
Is it hollering alot? Sick chicks holler more than the healthy ones, as if that is possible.
I wouldn't give them anything but chick starter, until you use up what you already have on hand.
Remember, chickens don't have any teeth, they need sand to eat to put in their craw for grinding up foods. I often sprinkle a bit of parakeet grit over their food, like salting your food. To ensure that they have a way to digest their food.
In about 2 weeks, you can offer the gravel separately, but watch them, you don't want to have a craw full of gravel and no food in there.
Once, you know they are eating a bit of gravel, then you can introduce new foods, just keep the bites really small. If bread, make sure it is pretty dry so they don't get choked on it.
I would add a drop or two of apple cider vinegar to their drinking water, to help them get their potassium levels up, which helps with muscles and energy.
And I would first feed them little bits of produce that you are eating. Nothing old, moldy, spoiled etc. They are babies after all.
But, if you eat an apple you can chop a bit up for them, from day one.
They need greens too. Just not all lettuce. Iceberg lettuce has vitamin k and not a whole lot of other vitamins, but other lettuces, and spinach... just stuff you eat.
Talk to your plants.... If your plants talk to you... Run!