Getting a child (w/ rigidbody) to follow parent (w/ rigidbody)

So, I have my parent object moving forward on it's local Z axis (in world it looks like its Z and Y since the obj is rotated). And also moves on the X local at times as well. I want the children to follow the parent, but also to be able to move independently.

Think of it as in the parent is a train car, and the child is on top of the train moving around.

right now the children won't follow the parent, when they both have rigidbodies. I had this working when the child was a rigidbody2D, but since I need rotations as well, I can't leave it as a 2D object.