Summary: In order to create Cross Account Bucket Access, you need to: 1) Create a Bucket, 2) Create a role in the child account in which the parent account will allow to assume the role int he parent account, 3) create the parent account with a trust that allows the child role to assume it, 4) attach an IAM policy to the parent role that provides access to the s3 bucket, and 5) Attach an IAM policy that allows the child role to assume the parent role.
