Can I add a rigidbody to the parents instead of every object?

Questions & Answers
1

I heard it’s bad for performance if you have moving colliders without rigidbody attached to them, and I should add rigidbodies setting them to kinematic. My question is, if I have many child objects with colliders, do I need to add a rigidbody to each, or can I add a rigidbody only to the parent?

2

Yes you can attach a Rigidbody to the parent object, and the colliders of the child objects will be automatically connected to the rigidbody of the parent object:

3

Maybe. With a rigidbody parent, you won’t be able to use raycasts or OnCollision to detect separate children.

All child colliders of an RB parent count as one big thing. Raycasts and OnCollision always just say they hit the parent. That’s perfect for most things (which is why it works that way.)

Ex:

empty "Cow"
  child box collider "head"
  child box collider "body"

Raycast to body and print hit.transform.name will tell you “Cow”