Is it necessary to attach a rigidbody/colliders to child objects?

Say for example you have a bullet as the parent object and it has a rigid body and some type of collider attach to it. This object has two child objects: The tip of the bullet and the shell casing. My question, is it necessary to also attach a rigid body and collider to each of these child objects or would a collider for each of these child objects suffice(Also would the parent object only have a rigid body or would it have both a rigid body and a collider)?

Depends on what you want. If you want the bullet to break in two pieces (the tip and the shell) when it hits something you will need the extra colliders and rigidbodys. In that case you will have to remove the collider and the rigidbody from the parent. However usually for a bullet the collider and rigidbody for the parent will be enough. You won’t need the extra colliders for your project i assume.