Targeting parts of a Prefab more than 1 level down?

Hello, I am trying to target part of a Prefab in one of my scripts. When I view the Prefab in the Project Explorer, I can "open" it to see one layer beneath, and I can click&drag these items to the inspector for use in my scripts. For some reason, I cannot target more than 1 level down. Is this expected behavior? I tried to solve this problem programmatically, by using a GetComponentInChildren() call on one of the items that I CAN see in the project explorer, but it is returning null. Please advise, thanks so much!