I seriously don't know for sure but my guess is that they may not be able to break the shells down in their crop. I would remove them the chick feed and chick grit(had to be chick grit its smaller) are fine.
The 2 month Olds should be in a coop outside and they don't need heat. The two week ones still need heat. I feel that the age difference might be part because of different needs.
What are you feeding them? What type of bedding are you using? How big is the brooder? Pictures of your set up...