Sounds like their crop. As they eat they store the food initially in their crop which is a pre-stomach organ that sits just to one side on their neck.
It will swell when they eat and then gradually go down as the crop starts to grind up the food and move it down the digestive track.
In small chicks it will look enormous when they feed - especially if they are eating a lot in a short amount of time if the feed ran out and then you give some and they decend like little vultures.
In older birds it is more hidden by their feathers.
Now, you'll get various advice about feeding meat birds but I'd recommend doing one of the following:
Keep the feeders filled 24 hours a day so that their feed is available continually - don't let the food run out before giving more food.
Others feed 12 hours on, 12 off - so put the feed down in the morning, keeping it full all day and then taking it up at night.
Personally I do the 24x7 feed access plan with mine, but even if you do the 12 on 12 off I'd not start taking the food up until the chicks were about 4 weeks old.
That's just my thoughts.... either way sounds like your chicks are just fine and it's their crops you are seeing.
