The ChildGameObjectsOnly attribute can be used on Components and GameObject fields and will prepend a small button next to the object-field that will search through all child gameobjects for assignable objects and present them in a dropdown for the user to choose from.
[ChildGameObjectsOnly]
public Transform ChildOrSelfTransform;
[ChildGameObjectsOnly]
public GameObject ChildGameObject;
[ChildGameObjectsOnly(IncludeSelf = false)]
public Light[] Lights;