collision.collider is the node that you collided with. If Bullet is the name of a child node of the node that you collide with your code should work. If Bullet is the node that you collided with then you don't need
.get("Bullet") . Check here if my wording was confusing.
I'm not sure what you are trying with split since nodes don't have a method named split. Are you perhaps trying to access a variable inside the node first, which happens to be a collection so you can split it?